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Firing-Associated Recycling of Coal-Fired Power Plant Fly Ash

Firing-Associated Recycling of Coal-Fired Power Plant Fly Ash Hindawi Journal of Analytical Methods in Chemistry Volume 2023, Article ID 8597376, 13 pages https://doi.org/10.1155/2023/8597376 Review Article 1 2 1 1 Vu Thi Ngoc Minh , Vuong-Hung Pham, Vu Hoang Tung, Cao Tho Tung, and Nguyen Thi Hong Phuong School of Chemical Engineering, Hanoi University of Science and Technology (HUST), No. 01, Dai Co Viet Road, Hanoi, Vietnam Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No. 01, Dai Co Viet Road, Hanoi, Vietnam Correspondence should be addressed to Vu Ti Ngoc Minh; minh.vuthingoc@hust.edu.vn Received 22 June 2022; Revised 18 September 2022; Accepted 7 October 2022; Published 27 February 2023 Academic Editor: Tien Duc Pham Copyright © 2023 Vu Ti Ngoc Minh et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Coal-fred power plant fy ash is a global environmental concern due to its small particle size, heavy metal content, and increased emissions. Although widely used in concrete, geopolymer, and fy ash brick production, a large amount of fy ash remains in storage sites or is used in landflls due to inadequate raw material quality, resulting in a waste of a recoverable resource. Terefore, the ongoing need is to develop new methods for recycling fy ash. Te present review diferentiates the physiochemical properties of fy ash from two coal combustion processes: fuidized bed combustion and pulverized coal combustion. It then discusses applications that can consume fy ash without strict chemical requirements, focusing on fring-associated methods. Finally, the challenges and opportunities of fy ash recycling are discussed. Environmental concerns arise from fuel extraction 1. Introduction operations, the coal power generation process, and Since the late 19th, coal-fred power has continuously emission treatment. Te primary discharge of coal grown and has been an essential source of global elec- combustion includes acidic gases such as SO , NO , and 2 x tricity. As of 2019, coal-fred power accounts for 36.7% of CO ; heavy metal vapors such as mercury; and solid the world’s total electricity production and 22.4% of the combustion residues such as dust, fy ash, and bottom ash. total electricity of countries in the organization for eco- Strongly acidic gases, including NO and SO , go through x 2 nomic cooperation and development (OECD) [1]. Due to chemical processes depending on the combustion system. environmental concerns and the need for sustainable Conventional boilers use low-ash coal grades and the development, coal-fred power has gradually been combustion chamber temperature is usually above replaced by other energy sources, such as wind, solar, 1400 C. At this temperature, nitrogen gas oxidizes, natural gas, biomass, and combustible waste power. forming NO in the fue gas. Te fue gas is directed to an However, under the pressure of economic development, adsorption tower that performs selective catalytic re- despite the decrease in proportion, the total capacity of duction (SCR) to convert NO into nitrogen gas and coal-fred power continues to grow, especially in Asian water. Te SO gas formed by the oxidation of sulfur- countries such as China, India, and Southeast Asia, containing substances is absorbed in a wet fue gas de- threatening net-zero targets by 2050. Te IEA predicts sulfurization (WFGD) system with lime/limestone slurry that coal-fred power generation from 2021 to 2024 will to produce calcium sulfate. Fluidized bed combustion increase by 4.1% in China, 11% in India, and 12% in boilers often use high-ash coal grades and the temperature Southeast Asia [2]. of the combustion chamber usually does not exceed 900 C 2 Journal of Analytical Methods in Chemistry to limit the generation of NO . In this system, limestone and technically efcient to some extent, these applications can be fed into the combustion chamber to absorb SO accumulate toxic substances on the fy ash, making post- gas [3]. adsorption treatment of fy ash even more complicated. Coal fy ash is also a primary environmental concern due Other applications of fy ash include raw materials for to its small particle size and heavy metal contents. Continued glass [33, 34], glass ceramics [35, 36], sintered bricks [37, 38], increases in coal-fred power generation have resulted in a and ceramic tiles [39, 40]. Tis review covers the physical continued increase in global coal ash emissions. While a and chemical properties of coal fy ash and focuses on the large amount of coal fy ash is used in nonfring construction fring-associated recycling of this material. Te main ob- materials such as concrete [4–6], fy ash bricks [7, 8], and jective of the review is to provide evidence that coal fy ash geopolymer [9, 10], a signifcant amount remains at the can be used as ceramic raw material without violating storage site or is used for landflls. A recent review of fy ash regulations on the release of heavy metals into the envi- usage in China shows that 56% of fy ash is used in con- ronment during use. Te review also proves that this method struction, 35% is used in landflls, and 9% is used in other can recycle a large amount of fy ash remaining in the storage applications [11]. sites because it does not place any limit on the origin and For safety reasons, fy ash used as pozzolanic additives quality of fy ash. in concrete and cement has stringent quality requirements. Te American standard ASTM C618-19 classifes fy ash 2. Methodology used for concrete into classes F and C by chemical com- position, loss-on-ignition, physical properties, and poz- Most of the documents cited in this review paper are zolanic activity [12]. Chemically, the most signifcant publications from Google Scholar and Web of Science da- diference between Class C and Class F is in the CaO tabases. Te statistical data from the International Energy content and the total SiO + Al O + Fe O content. Due to Agency (IEA) and the India Central Electricity Authority 2 2 3 2 3 its high CaO content, often associated with lime and cal- (CEA) and test methods of the American Society for Testing cium sulfate, Class C fy ash features self-adhesive prop- and Materials (ASTM) and the International Standards erties [13]. Class F fy ash contains a higher SiO content Organization (ISO) are also cited. than Class C and shows pozzolanic activity in mortar and As a supplement to the review, coal fy ash samples were collected and characterized. Te samples were collected from concrete. Ones that do not belong to Class C and Class F are not considered pozzolanic additives. Due to the quality four coal-fred power plants in Vietnam: Dong Trieu, Mong requirements mentioned above, a large portion of fy ash Duong I, Uong Bi, and Ninh B`ınh. Dong Trieu and Mong piles up in storage sites by the power plant or is used for Duong I power plants apply the fuidized bed combustion landflls and mine backflls [14, 15]. (FBC) technique, whereas Uong Bi and Ninh Binh power Under environmental stress, intensive studies have fo- plants employ the pulverized coal combustion (PCC) cused on thoroughly treating and utilizing fy ash in more technique. Te fy ash was dried overnight at 115 C before value-added applications. In India, 32.87% of fy ash was left being characterized by X-ray difractometry, scanning untreated during 2017–2018 [16], which decreased to 7.59% electron microscopy, and laser scattering particle size by 2020–2021 [17]. During the year 2020–2021, ten modes of analysis. fy ash utilization were efective: cement (25.81%), mine A Bruker X-ray Difractometer model D2 Phaser was flling (6.20%), bricks and tiles (12.98%), reclamation of low- used for phase analysis. Care was taken to keep the top lying area (15.59%), ash dyke raising (7.94%), roads and surface of the sample mass fat and on the plane of the top of fyovers (15.04%), agriculture (0.03%), concrete (0.83%), the sample holder to avoid possible displacement errors. Te hydropower sector (0.03%), and others (7.97%) [17]. In this copper radiation source of the instrument was operated at example, the main distribution of fy ash is in construction, 30 kV and 20 mA while scanning between 10 and 60 degrees of which the portion used for landfll and foundation rec- 2-θ. Te data collected from the difractometer were ana- lamation accounts for a greater proportion than that used in lyzed using Crystal Impact’s Match software using the COD cement, concrete, brick, and tiles. Te environmental con- database. cerns associated with such applications include wind-blown Te microstructural properties of fy ash were investi- dust [18] and the dissolution of toxic metals into ground- gated by a JOEL 6360LV scanning electron microscope. Fly water [19, 20]. Terefore, methods of using fy ash to mit- ash samples were sprinkled onto PELCO Tabs carbon igate these risks are still under study. conductive tabs adhered to the SEM sample holders, which Due to its porous structure, fy ash has a large specifc were then gently tapped to remove nonadherent particles. surface area, which is suitable as a substrate in waste gas and Before being placed in the SEM instrument, the sample wastewater treatment. In gas treatment, fy ash is mixed with holders were placed in a sputter coater to coat the fy ash calcium hydroxide to adsorb SO in fue gas [21, 22]. Tis particles with a thin layer of platinum for conductive pur- mixture can also absorb toxic organic vapors such as toluene poses. Te SEM ran under a vacuum. [23] and m-xylene [24]. In wastewater treatment, fy ash Te fy ash particle size distribution was analyzed using a improves the precipitation of heavy metals [25], boron [26], HORIBA Partica Mini LA350 instrument, which used a laser and phosphate [27] in the presence of calcium hydroxide. In source with a wavelength of 650 nm. Te instrument works addition, fy ash can adsorb phenolic compounds [28, 29], on the principle of laser difraction and Mie’s light scattering dyes [30, 31], and pesticides [32]. Although economically theory. Journal of Analytical Methods in Chemistry 3 composition of Al Fe Ti Si O [56]. FBC boilers 3. Properties of Coal-Fired Power Plant Fly Ash 4.61 0.05 0.02 1.32 9.66 operate at approximately 850 C, which is not high enough Coal combustion techniques signifcantly infuence the for mullite formation; thus, no difraction peaks of mullite morphology, particle shape, and particle size distribution of can be observed. Instead, hematite, phengite and anhydrite fy ash. Figure 1 presents the scanning electron microscopy gehlenite might be present in the XRD patterns of FBC fy image of fy ash from four coal-fred thermal power plants. ash [4, 55, 57]. Figure 3 presents the XRD patterns of four Due to high-temperature (>1400 C) combustion, the fux types of fy ash. PCC fy ash shows only two crystal phases, agents in the PCC fy ash melted, stuck to unmelted particles, quartz and mullite, while FBC fy ash shows three crystalline and rounded under surface tension, forming spherical phases, including quartz, phengite, and hematite. particles upon cooling. On the other hand, FBC fy ash In addition to oxide solids, fy ash may contain signif- particles have irregular shapes as they were created from a icant amounts of unburnt carbon (UC), a result of in- low-temperature (<900 C) combustion process and thus did complete combustion. Te UC content depends on the not have sufcient liquid phase to form spherical particles. combustion technology, operating conditions, and coal type. Fly ash is a collection of fne particles that have harmful It is assessed by the loss-on-ignition (LOI) test [58]. Termal efects on human health and ecology. Signifcantly, the wind analysis of fy ash confrms the presence of UC with a large can blow fne fy ash particles long distances, making fy ash a exothermic peak incorporated with weight loss. Te starting global health and environmental concern [41, 42]. Most coal point of UC combustion in TG-DTA analysis varies with the fy ash has particle sizes under 100 micrometers [43–45]. type of coal fy ash [55, 59, 60]. Typically, the amount of UC Figure 2 shows the particle size distribution curves of four in fy ash is higher than that in bottom ash [61]. types of fy ash. Tey present the frequency distribution (q in Fly ash UC is an inexpensive source of activated carbon %) of particles within a specifc size range. Although the for adsorbing harmful components of fue gas such as NO particle size distribution curves are diferent and the com- [62], mercury vapor [63, 64], and organic substances [65]. bustion methods show no trend in these curves, they all have However, UC is a factor hindering the use of fy ash in many one thing in common: 90% of each sample (D90) is smaller construction applications. From the concrete perspective, than 100 micrometers and the median particle size (D50) is UC adsorbs air-entraining admixture, reducing the physical in the range of 12.4–17.6 micrometers. properties of concrete [66]. As specifed by ASTM C618 - 19 Te chemical composition of fy ash is quite complicated, standard, the LOI content of fy ash used as pozzolanic depending on the coal origin and the combustion technique. additives in concrete should be under 6% with some tol- By weight percentage, dominant in fy ash is SiO , followed 2 erance to the Class F fy ash [12]. by Al O , and other oxides such as Fe O , CaO, K O, Na O, 2 3 2 3 2 2 Te leaching test performed on fy ash by the EN and TiO (Table 1). Te introduction of limestone to the FBC 2 12457–4 test method shows that the heavy metal concen- boilers adds CaO to the solid combustion residues. Fly ash is trations in the leachate are below the inert waste limits also a source of heavy metals, including but not limited to V, specifed by the European Landfll Directive [67]. However, Cr, Mn, Co, Ni, As, and Hg [46, 48, 49]. In addition, there are the long-term leaching test indicates a potential risk to soil distinct diferences between the chemical composition of and groundwater [68]. As has been demonstrated in various diferent size fractions of fy ash from a boiler. Signifcantly studies, it is possible to immobilize heavy metals of solid higher concentrations of total carbon and heavy metals are wastes from wastewater treatment plants [4, 69], red sludge found in smaller particles. Slightly higher concentrations of from bauxite processing plants [70], and blast furnace slag major elements, Si, Al, Fe, Ca, K Na, Mg, Mn, and Ti, are [71–73] in the structure of glass [74], glass ceramics [75], and present in coarser ones [46, 48]. Fortunately, most heavy ceramic materials [76]. Likewise, introducing fy ash to metals are locked in the glass phase of fy ash [50]. However, ceramic production is a potential solution to lock the heavy the ability to release heavy metals and rare Earth elements metals of fy ash in the ceramic network. from fy ash in landflls is always a community concern. Te Due to high SiO and Al O contents, fy ash is suitable 2 2 3 allowable limits for the disposal of coal combustion residuals as a precursor for aluminosilicate crystalline phases, such as vary between countries and regions but may be tightened in mullite [77, 78] and cordierite [79–82]. In addition, the high the future [51, 52]. CaO content in certain types of fy ash, especially those from Te mineral composition of fy ash depends on the coal FBC boilers using limestone to absorb SO , makes them type and combustion techniques. X-ray difraction (XRD) suitable as a raw material for glass ceramics that contain analysis of a series of fy ash showed a signifcant variation in calcium silicate and calcium aluminosilicate crystalline the glass phase content, from 45 to 80% [53]. Besides the phases [83–86]. Furthermore, due to its high glass phase glass phase, quartz is the most popular crystalline phase. Its content, fy ash is also considered a fuxing agent that difraction peaks stand out in the XRD patterns of all types of promotes sintering in ceramics. coal fy ash. Since PCC boilers operate at temperatures above 1400 C, high enough for mullite crystallization, its fy ash 4. Firing-Associated Recycling of Fly Ash often contains mullite (3Al O .2SiO ) [54, 55]. Te mineral 2 3 2 composition of mullite is heterogeneous due to the sub- 4.1. Bricks. Fly ash can replace up to 80% of clay in clay stitution of impurities at the aluminum sites, forming solid bricks, but some adjustments should be made. Te presence solutions. XRD and nuclear magnetic resonance analysis on of a large amount of fy ash signifcantly reduces the raw mix a bituminous coal fy ash showed a mullite average chemical plasticity index. Terefore, it is necessary to add plastic 4 Journal of Analytical Methods in Chemistry (a) (b) (c) (d) Figure 1: Scanning electron microscopy images of fy ash: (a) Uong Bi PCC, (b) Ninh Binh PCC, (c) Dong Trieu FBC, and (b) Mong Duong I FBC. additives to facilitate extrusion. In addition, the porous threshold limits of EPA Victoria (Australia) for industrial structure of fy ash particles impedes the sintering process, solid waste. A study by Leiva et al. also shows that the increasing the porosity of the sintered product even at low leaching contents are below the limits of the European degrees of replacement [87]. Hence, it may be necessary to Landfll Directive for granular waste, the Italian national grind the fy ash to break down its porous structure and regulation for the reuse of waste in construction material, increase the contact surface area to speed up the sintering and the Dutch Soil Quality Decree for bound or shaped process [37]. Along with the high porosity, the high re- materials [47]. fractoriness of fy ash requires an increase of the fring temperature by 50 to 100 C so that the physical properties of 4.2. Ceramic Tiles. Fly ash with high fux the sintered products meet those of conventional clay bricks. Most reports indicate that when using large amounts of fy (Fe O + Na O + K O) contents can be used to replace 2 3 2 2 feldspar in ceramic tile production. Olgun et al. were able to ash, the fring temperature is 1050 C or higher [37, 88]. A test at the industrial scale reveals a number of obstacles that improve the modulus of rupture of wall tiles by replacing potassium feldspar in the raw mix with fy ash and borax should be overcome: brick swelling and deformation due to local melting and the occurrence of black core and cracks solid waste without having to change the sintering tem- perature [90]. As the porous structure of fy ash hinders [89]. It is possible to manufacture bricks with 100% of the sintering, resulting in high porosity, it is mainly used as a raw material for wall tiles, equivalent to Group BIII as classifed solid material being fy ash. In this case, shaping requires a temporary binder because most fy ash lacks adhesive ca- by EN 14411. Tis type of ceramic tile is not subject to the loading force in use but requires high water absorption for pacity. Kayali reported that although the apparent density of good mortar adhesion, allowing higher water absorption and the fred product is 28% lower than that of conventional clay bricks, its compressive strength is 24% higher, and brick-to- lower fexural strength than those of the foor tiles [91]. Similar to that observed in clay bricks, the disruption of mortar bond strength is 44% higher than those of the best local clay bricks [8]. porous fy ash particles also improves the sintering of the ceramic tiles [92]. When high alumina fy ash is used and a It is noteworthy that heavy metals are immobilized in the ceramic structure of the brick. Sutcu et al. [88] showed that foor tile is the desired product, the fring temperature must be increased to achieve the required physical properties. In the release of heavy metals, including Cr, Mn, Ni, Cu, Zn, As, Cd, Ba, Hg, and Pb, was substantially lower than the particular, the sintering process becomes more difcult when the Al O content is up to 40%, equivalent to freclay maximum allowable limits regulated by U.S. Environmental 2 3 Protection Agency (EPA) for hazardous solid waste and the refractory bricks. Wang et al. showed that for producing Journal of Analytical Methods in Chemistry 5 Uong Bi PCC Ninh Binh PPC Dong Trieu FBC Mong Duong I FBC 0.1 1 10 100 1000 Diameter (μm) Figure 2: Fly ash particle size distribution. Table 1: Chemical composition of fy ash. Chemical Ninh Binh, Mong Duong I, Seyitomer, Tennessee, US XianYang, Bokaro, Bokaro, Spain [47] a a b,d c,d composition Vietnam Vietnam Turkey [7] [15] China [33] India [46] India [46] SiO (wt.%) 45.20 52.16 57.21 49.54 54.39 39.57 51.94 45.02 Al O 2 3 19.99 23.65 20.39 19.11 23.21 22.13 26.41 25.18 (wt.%) Fe O 2 3 6.86 5.97 10.89 14.79 11.50 3.21 5.21 16.52 (wt.%) CaO (wt.%) 1.52 1.94 2.75 6.23 4.77 0.42 0.49 8.53 MgO (wt.%) 1.29 1.09 4.96 0.42 1.73 0.19 0.27 0.97 K O (wt.%) 3.13 3.30 1.36 — 1.32 0.93 0.95 2.14 Na O (wt.%) 0.32 0.19 0.40 — 0.37 0.13 0.16 0.43 TiO (wt.%) 0.57 0.91 0.81 — — 2.08 2.09 — SO — — — 0.34 — 1.45 0.50 — LOI (wt.%) 20.15 9.46 0.94 2.43 13.38 29.8 9.67 0.7 Cd (ppm) — — — — — — — 0.9 Pb (ppm) — — 79.0 — — 31.06 63.42 58 Zn (ppm) — — 112.6 — — 47.2 76.6 309 Cu (ppm) — — 98.8 — — 73.8 97.7 - Cr (ppm) — — 454.5 — — 140.7 137.4 149 Ni (ppm) — — 1975.9 — — 77.5 65.7 74 Mn (ppm) — — 790.4 — — 117.8 268.4 - a b c Present work, by chemical analysis. Fraction retained by Sieve No. 150 BS (+104 μm). Fraction passed through by Sieve No. 300 B.S and retained by Sieve No 350 BS (−53 μm to + 45 μm). Incomplete list of the analyzed heavy metals and rear Earth elements from the cited sources. q (%) q (%) q (%) q (%) 6 Journal of Analytical Methods in Chemistry Q Q Q Q Q H Q Mong Duong I FBC Dong Trieu FBC Cam Pha H M Uong Bi PCC 10 20 30 40 50 60 2-theta (deg.) Q: Quartz P: Phengite M: Mullite H: Hematite Figure 3: Te X-ray difraction patterns of fy ash samples. ceramic tiles with water absorption below 0.5% from a raw concentrations in the leachate of ceramic tiles. However, one mix with 70% high alumina fy ash, the sintering temper- can infer from the studies on bricks that ceramic tiles are also ature is as high as 1300 C [93]. Solid wastes with high fux capable of immobilizing hazardous elements, even better contents, such as glass waste [94, 95], boron waste [90, 96], than bricks because of their denser structure formed by or lithium mine tailings, are viable additives that help lower higher sintering temperatures. the sintering temperature [97]. Wang et al. used high alumina content fy ash (40% 4.3. Insulating Materials and Lightweight Concrete Aggregates. Al O ) and waste glass to make insulating ceramic tiles, 2 3 consisting of a layer of foam and a layer of dense material. Insulating materials and lightweight concrete aggregates feature porous structures constructed by closed pores. Te Te raw mix of the foam layer consists of waste glass (50%), sintering of these materials requires a simultaneous gen- fy ash (30%), clay (15%), and feldspar (5%). Te raw mix of the dense layer consists of fy ash (60–80%), clay (15%), and eration of gas and molten phases with sufcient quantity and viscosity. Te molten phase entraps the gas, forming closed quartz (5–25%). With single-loading pressing and calcina- tion at 1200 C, the foam layer performs insulation capacity pores upon cooling. Fly ash is an excellent candidate for this application due to its UC and high glass phase contents. Te with an average pore diameter of 300–500 micrometers. In addition, the bi-layer ceramic shows a fexural strength of UC oxidizes at elevated temperatures releasing carbon di- oxide gas while the glass phase is ready to melt. However, if 31.5 MPa. Tis way, over 70 weight percent of the tile originates from industrial waste [94]. fy ash is the only raw material, the fring temperature should be up to 1300–1400 C for bloating [100]. Furthermore, UC Most fy ash has a Fe O content of 4–10% (Table 1). Te 2 3 presence of a remarkably high Fe O content gives a clay might be low in certain fy ash sources (Table 1). Terefore, 2 3 fuxes and gas-forming agents are required for (1) generating brick color to fy ash-based ceramic bodies [8]. On the other gases, (2) reducing the fring temperature, and (3) adjusting hand, the combination of Fe O and UC causes the black 2 3 core in bricks [98]. Terefore, if masking the ceramic body the quantity and viscosity of the liquid phase for entrapping the generated gases. Te fuxes of interest include waste glass color is desired, an engobe glaze with high opacity and whiteness should be applied on the surface of the tiles before [101, 102], limestone [103, 104], and synthetic chemicals such as sodium salts [105, 106]. Some fuxes, including the decoration is fnished. Although a test method for determining lead and cad- limestone and soda, also play the role of gas-forming agents. Since most fy ash is not self-adhesive, a temporary mium leaching from ceramic tiles is available [99], there is no harmonized requirement for hazardous element binder is required to form the green pellets. On a laboratory Intensity (a.u.) Journal of Analytical Methods in Chemistry 7 scale, organic binders such as polyvinyl alcohol have been et al. fabricated porous materials from coal mine waste tested [100]. On a larger scale, clay is a popular binder due to kaolin and spherical hollow fy ash, using bentonite as the its availability and low cost [103, 106]. Besides, bentonite binder and calcium iodate (Ca(IO ) .6H O) as a focculant. 3 2 2 [102, 104] and ordinary Portland cement [104] have been After fring at 1550 C, the resulting product has a porosity of used. 44.73–46.12%, with 99% of the porosity being closed pores, and mullite is the main crystalline phase. Te freeze-gel casting/polymer sponge technique can produce a porous 4.4. Mullite and Cordierite Ceramics. Fly ash is also a raw mullite ceramic with a porosity of 66.1% and an average material for the production of mullite ceramics due to its high compressive strength of 45 MPa [112]. SiO and Al O contents. Low alumina fy ash often requires an Fly ash is also suitable as a raw material for cordierite 2 2 3 alumina augmenting agent such as aluminum oxide [107], ceramics, a material well known for its low LTEC. Cordierite bauxite [77, 108], or high alumina industrial wastes [78, 109]. crystallizes in the orthorhombic system. Its dimorphism is Unlike FBC fy ash, mullite is usually available in PPC fy ash. hexagonal indialite which is more refractory than cordierite Te presence of mullite makes the mullite crystallization in PCC [113–115]. Although the IMA formula of cordierite is fy ash-based ceramics easier than in those using FBC fy ash. 2MgO.2Al O .5SiO , it appears as a solid solution in practice 2 3 2 Although fuxes are available in fy ash and start forming [116, 117]. Tanks to its low LTEC, cordierite ceramic is a liquid phase at relatively low temperatures, fy ash-based highly resistant to thermal shock. Tis material also pos- mullite ceramics are often sintered at high temperatures, sesses high chemical resistance and a low dielectric constant. usually above 1400 C, because mullite is a refractory phase. Te synthesis of cordierite from fy ash requires MgO Making dense mullite ceramics from PCC fy ash and supplementing materials such as talc [118], magnesia [119], bauxite, Dong et al. showed that the solid-state reaction magnesite [120], or dolomite [121]. Unlike mullite, the between cristobalite and corundum occurs at temperatures sintering temperature of cordierite ceramics is usually below ° ° below 1300 C, followed by the dissolution of the corundum 1200 C. In addition to indialite, fy ash-based cordierite into the liquid phase at higher temperatures where sec- ceramics may contain mullite, cristobalite, periclase, and ondary crystallization occurs. Te formation and recrys- spinel [118, 120]. Porous cordierite ceramics from fy ash can tallization of mullite lead to volume expansion which is be used for microfltration membranes [82], catalytic sub- slightly dominant over the shrinkage of the sintering pro- strates [118], and membrane supports [121]. cess. At 1600 C, the material achieves a relative density of 93.94% with spherical pores and fracture strength of 186.19 MPa [77]. Jung et al. combined aluminum oxide with 4.5. Foam Glass and Glass Ceramics. Tanks to its high PCC fy ash, from which UC was removed, to produce silicate glass content, fy ash can be used as a raw material for mullite ceramics at temperatures of 1400, 1500, and 1600 C. glass and glass-ceramic synthesis. Erol et al. produce glass by Although the pellet is cold isostatic pressing, the density of melting fy ash at 1500 C, glass ceramics by annealing the the fred product was relatively low (63%). Tis low density is obtained glass at 1150 C, and ceramics by fring green pellets due to the exaggerated grain growth of needle-shaped at 1200 C. Te only phase in the glass products is amor- mullite crystals incorporated with voids formation [107]. phous, the crystalline phase in the glass-ceramic products is 3+ Te introduction of the supplements alters the sintering augite (Ca(Mg, Fe , Al) (Si, Al) O ), and in the ceramic is 2 6 mechanism either by preventing excessive grain growth or quartz, mullite, and enstatite ((Mg, Fe)SiO ) [34]. However, by creating new phases, thereby improving the physical making glass and glass ceramics products from fy ash can be properties of the sintered products. Te presence of 3Y-PSZ difcult due to high SiO and Al O (network formers) 2 2 3 inhibits the crystal growth of mullite, leading to an improved contents and insufcient fuxes (network modifers). fracture strength [107]. Magnesia efectively promotes sin- Terefore, it is necessary to add fux ingredients to lower the tering, signifcantly above 1450 C. It slightly reduces the melting temperature. linear thermal expansion coefcient (LTEC) at 1300 C by One of the most attentive applications of fy ash-based forming low thermal expansion α-cordierite. However, it glass is glass foam, which is considered a low-cost insulation slightly increases the LTEC above 1400 C due to the for- material in construction. Besides fy ash, glass foam raw mixes mation of high expansion corundum and the spinel often have fux and foaming agents. Most of them are in- (MgAl O ) [108]. SiC enables the growth of mullite needle- dustrial wastes. Te commonly used fux is waste glass. It is 2 4 shaped crystals out of the saturated glass phase, resulting in used in large quantities comparable to fy ash [33, 122]. Al- better bonding of the crystals, reducing the true porosity but ternatively, borax [122] and soda [123] can be added to the raw increasing the closed-pore porosity. As a result, thermal mix as network former and modifer, respectively. Foaming conductivity and cold crushing strength are improved [110]. agents can be obtained from a variety of sources, including TiO hinders the sintering process at low temperatures but calcium carbonate [122], soda [123], and SiC [33, 124]. For promotes sintering above 1300 C. Tis phenomenon is instance, Bai et al. produced glass foam by melting a raw mix of useful for the unsaturated sintering of porous mullite ce- fy ash, waste glass, and SiC waste at 950 C. Te resulting ramic membrane supports [111]. product expanded 5.81 times compared to the green body [33]. Te porous structure of fy ash particles and the re- Another fascinating application of fy ash-based glass is crystallization of mullite favor the fabrication of porous glass ceramics because of their high mechanical strength and mullite ceramics for insulation or fltration purposes. Chen thermal shock resistance. Te crystalline phase in fy ash- 8 Journal of Analytical Methods in Chemistry based glass ceramics is controlled by adjusting the com- 6. Conclusion position of the raw mix. Te two glass ceramic systems of Coal fy ash is a global concern due to its small particle size great interest are SiO2-Al2O3-CaO [125, 126] and SiO - and heavy metal contents with increasing emissions. Faced Al O -MgO [79, 127]. When a high amount of Fe O is 2 3 2 3 with concerns about wind-blowing dust and heavy metal available in fy ash, the system SiO -Al O -Fe O -CaO is of 2 2 3 2 3 leakage from fy ash landflls and storage sites, eforts to use interest [128]. Depending on the chemical composition of fy ash for safe and value-added applications are underway. the mix, the crystalline phases in the SiO -Al O -CaO glass- 2 2 3 Notable among them are fring-associated measures, in- ceramics include diopside (Ca(Mg, Al) (Si, Al) O ) [125], 2 6 cluding the production of ceramics, lightweight concrete augite (Ca(Mg, Fe)Si O ) [34, 125], and wollastonite 2 6 aggregates, glass, and glass ceramics. Tere is almost no (CaSiO ) [129]. SiO -Al O -MgO glass ceramics are of the 3 2 2 3 restriction for fy ash in these applications. Most notably, the most interest because they contain cordierite, a mineral with recycling of fy ash and other industrial solid wastes can be low LTEC, and are suitable for thermal shock applications combined in these applications without violating regulations [79, 127]. on heavy metals released during use. Besides, the unburnt carbon content can alleviate the heat required for fring. Te 4.6. Portland Cement Clinker. In cement production, fy ash fring temperature, product phase composition, and physical often serves as a pozzolanic additive, but high UC fy ash is properties of the products can be controlled by supple- not suitable for this application. Another possibility of menting agents. However, the sulfur content might be high using fy ash in cement production is to replace clay in the in certain types of fy ash and go into the fue gas as sulfur raw mixes of Portland cement clinker [130–133], belite dioxide. Tis problem can be solved in cement kilns but cement clinker [134], and belite-sulfoaluminate cement should be addressed in other applications. clinker [135]. Tis perspective has been investigated from the laboratory to the industrial scale. Laboratory studies Data Availability showed that the use of fy ash reduced the sintering All data generated and analyzed to support the fndings of temperature of Portland cement clinker [130, 131]. this study are available within the article. Commercial demonstration on high carbon fy ash showed that the obtained fy ash-based Portland cement Conflicts of Interest performs a higher compressive strength than the normal ones et al.l ages, despite a lower fneness [132]. Te use of Te authors declare that they have no conficts of interest. high carbon high fy ash has the additional beneft of fuel- saving [133]. Authors’ Contributions Unlike ceramic fring, SO formed in the fring process of clinker can be reabsorbed by calcium oxide. Te resulting Conceptualization and supervision were done by Vu Ti calcium sulfate helps reduce the gypsum amount required Ngoc Minh. Vu Ti Ngoc Minh reviewed the article. Data for cement setting control [136]. Terefore, using fy ash as accuracy was tested by Pham Hung Vuong. Pham Hung raw material for cement clinker brings valuable economic, Vuong reviewed the article. Conceptualization and data technical, and environmental benefts. accuracy were done by Vu Hoang Tung. Data analysis was performed by Cao To Tung. Cao To Tung reviewed the 5. Challenges and Opportunities article. Nguyen Ti Hong Phuong performed data analysis. Although it is demonstrated that fy ash-based glass and Acknowledgments ceramic products can immobilize the heavy metals and rare elements of fy ash in their oxide networks, they are not Tis work was fnancially supported by the Vietnam Min- suitable as food and drug containers. Tis restriction is istry of Training and Education under grant number B2020- mainly due to the stringent regulations applied to these BKA-17. Te authors wish to thank Mr. Mai Van Vo and Ms. products [137–139]. On the other hand, SO emissions x Bui Ti Tu Ha for their valuable technical support. from burning fy ash are also of concern because the SO content in certain types of fy ash may be high. 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Firing-Associated Recycling of Coal-Fired Power Plant Fly Ash

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10.1155/2023/8597376
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Hindawi Journal of Analytical Methods in Chemistry Volume 2023, Article ID 8597376, 13 pages https://doi.org/10.1155/2023/8597376 Review Article 1 2 1 1 Vu Thi Ngoc Minh , Vuong-Hung Pham, Vu Hoang Tung, Cao Tho Tung, and Nguyen Thi Hong Phuong School of Chemical Engineering, Hanoi University of Science and Technology (HUST), No. 01, Dai Co Viet Road, Hanoi, Vietnam Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No. 01, Dai Co Viet Road, Hanoi, Vietnam Correspondence should be addressed to Vu Ti Ngoc Minh; minh.vuthingoc@hust.edu.vn Received 22 June 2022; Revised 18 September 2022; Accepted 7 October 2022; Published 27 February 2023 Academic Editor: Tien Duc Pham Copyright © 2023 Vu Ti Ngoc Minh et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Coal-fred power plant fy ash is a global environmental concern due to its small particle size, heavy metal content, and increased emissions. Although widely used in concrete, geopolymer, and fy ash brick production, a large amount of fy ash remains in storage sites or is used in landflls due to inadequate raw material quality, resulting in a waste of a recoverable resource. Terefore, the ongoing need is to develop new methods for recycling fy ash. Te present review diferentiates the physiochemical properties of fy ash from two coal combustion processes: fuidized bed combustion and pulverized coal combustion. It then discusses applications that can consume fy ash without strict chemical requirements, focusing on fring-associated methods. Finally, the challenges and opportunities of fy ash recycling are discussed. Environmental concerns arise from fuel extraction 1. Introduction operations, the coal power generation process, and Since the late 19th, coal-fred power has continuously emission treatment. Te primary discharge of coal grown and has been an essential source of global elec- combustion includes acidic gases such as SO , NO , and 2 x tricity. As of 2019, coal-fred power accounts for 36.7% of CO ; heavy metal vapors such as mercury; and solid the world’s total electricity production and 22.4% of the combustion residues such as dust, fy ash, and bottom ash. total electricity of countries in the organization for eco- Strongly acidic gases, including NO and SO , go through x 2 nomic cooperation and development (OECD) [1]. Due to chemical processes depending on the combustion system. environmental concerns and the need for sustainable Conventional boilers use low-ash coal grades and the development, coal-fred power has gradually been combustion chamber temperature is usually above replaced by other energy sources, such as wind, solar, 1400 C. At this temperature, nitrogen gas oxidizes, natural gas, biomass, and combustible waste power. forming NO in the fue gas. Te fue gas is directed to an However, under the pressure of economic development, adsorption tower that performs selective catalytic re- despite the decrease in proportion, the total capacity of duction (SCR) to convert NO into nitrogen gas and coal-fred power continues to grow, especially in Asian water. Te SO gas formed by the oxidation of sulfur- countries such as China, India, and Southeast Asia, containing substances is absorbed in a wet fue gas de- threatening net-zero targets by 2050. Te IEA predicts sulfurization (WFGD) system with lime/limestone slurry that coal-fred power generation from 2021 to 2024 will to produce calcium sulfate. Fluidized bed combustion increase by 4.1% in China, 11% in India, and 12% in boilers often use high-ash coal grades and the temperature Southeast Asia [2]. of the combustion chamber usually does not exceed 900 C 2 Journal of Analytical Methods in Chemistry to limit the generation of NO . In this system, limestone and technically efcient to some extent, these applications can be fed into the combustion chamber to absorb SO accumulate toxic substances on the fy ash, making post- gas [3]. adsorption treatment of fy ash even more complicated. Coal fy ash is also a primary environmental concern due Other applications of fy ash include raw materials for to its small particle size and heavy metal contents. Continued glass [33, 34], glass ceramics [35, 36], sintered bricks [37, 38], increases in coal-fred power generation have resulted in a and ceramic tiles [39, 40]. Tis review covers the physical continued increase in global coal ash emissions. While a and chemical properties of coal fy ash and focuses on the large amount of coal fy ash is used in nonfring construction fring-associated recycling of this material. Te main ob- materials such as concrete [4–6], fy ash bricks [7, 8], and jective of the review is to provide evidence that coal fy ash geopolymer [9, 10], a signifcant amount remains at the can be used as ceramic raw material without violating storage site or is used for landflls. A recent review of fy ash regulations on the release of heavy metals into the envi- usage in China shows that 56% of fy ash is used in con- ronment during use. Te review also proves that this method struction, 35% is used in landflls, and 9% is used in other can recycle a large amount of fy ash remaining in the storage applications [11]. sites because it does not place any limit on the origin and For safety reasons, fy ash used as pozzolanic additives quality of fy ash. in concrete and cement has stringent quality requirements. Te American standard ASTM C618-19 classifes fy ash 2. Methodology used for concrete into classes F and C by chemical com- position, loss-on-ignition, physical properties, and poz- Most of the documents cited in this review paper are zolanic activity [12]. Chemically, the most signifcant publications from Google Scholar and Web of Science da- diference between Class C and Class F is in the CaO tabases. Te statistical data from the International Energy content and the total SiO + Al O + Fe O content. Due to Agency (IEA) and the India Central Electricity Authority 2 2 3 2 3 its high CaO content, often associated with lime and cal- (CEA) and test methods of the American Society for Testing cium sulfate, Class C fy ash features self-adhesive prop- and Materials (ASTM) and the International Standards erties [13]. Class F fy ash contains a higher SiO content Organization (ISO) are also cited. than Class C and shows pozzolanic activity in mortar and As a supplement to the review, coal fy ash samples were collected and characterized. Te samples were collected from concrete. Ones that do not belong to Class C and Class F are not considered pozzolanic additives. Due to the quality four coal-fred power plants in Vietnam: Dong Trieu, Mong requirements mentioned above, a large portion of fy ash Duong I, Uong Bi, and Ninh B`ınh. Dong Trieu and Mong piles up in storage sites by the power plant or is used for Duong I power plants apply the fuidized bed combustion landflls and mine backflls [14, 15]. (FBC) technique, whereas Uong Bi and Ninh Binh power Under environmental stress, intensive studies have fo- plants employ the pulverized coal combustion (PCC) cused on thoroughly treating and utilizing fy ash in more technique. Te fy ash was dried overnight at 115 C before value-added applications. In India, 32.87% of fy ash was left being characterized by X-ray difractometry, scanning untreated during 2017–2018 [16], which decreased to 7.59% electron microscopy, and laser scattering particle size by 2020–2021 [17]. During the year 2020–2021, ten modes of analysis. fy ash utilization were efective: cement (25.81%), mine A Bruker X-ray Difractometer model D2 Phaser was flling (6.20%), bricks and tiles (12.98%), reclamation of low- used for phase analysis. Care was taken to keep the top lying area (15.59%), ash dyke raising (7.94%), roads and surface of the sample mass fat and on the plane of the top of fyovers (15.04%), agriculture (0.03%), concrete (0.83%), the sample holder to avoid possible displacement errors. Te hydropower sector (0.03%), and others (7.97%) [17]. In this copper radiation source of the instrument was operated at example, the main distribution of fy ash is in construction, 30 kV and 20 mA while scanning between 10 and 60 degrees of which the portion used for landfll and foundation rec- 2-θ. Te data collected from the difractometer were ana- lamation accounts for a greater proportion than that used in lyzed using Crystal Impact’s Match software using the COD cement, concrete, brick, and tiles. Te environmental con- database. cerns associated with such applications include wind-blown Te microstructural properties of fy ash were investi- dust [18] and the dissolution of toxic metals into ground- gated by a JOEL 6360LV scanning electron microscope. Fly water [19, 20]. Terefore, methods of using fy ash to mit- ash samples were sprinkled onto PELCO Tabs carbon igate these risks are still under study. conductive tabs adhered to the SEM sample holders, which Due to its porous structure, fy ash has a large specifc were then gently tapped to remove nonadherent particles. surface area, which is suitable as a substrate in waste gas and Before being placed in the SEM instrument, the sample wastewater treatment. In gas treatment, fy ash is mixed with holders were placed in a sputter coater to coat the fy ash calcium hydroxide to adsorb SO in fue gas [21, 22]. Tis particles with a thin layer of platinum for conductive pur- mixture can also absorb toxic organic vapors such as toluene poses. Te SEM ran under a vacuum. [23] and m-xylene [24]. In wastewater treatment, fy ash Te fy ash particle size distribution was analyzed using a improves the precipitation of heavy metals [25], boron [26], HORIBA Partica Mini LA350 instrument, which used a laser and phosphate [27] in the presence of calcium hydroxide. In source with a wavelength of 650 nm. Te instrument works addition, fy ash can adsorb phenolic compounds [28, 29], on the principle of laser difraction and Mie’s light scattering dyes [30, 31], and pesticides [32]. Although economically theory. Journal of Analytical Methods in Chemistry 3 composition of Al Fe Ti Si O [56]. FBC boilers 3. Properties of Coal-Fired Power Plant Fly Ash 4.61 0.05 0.02 1.32 9.66 operate at approximately 850 C, which is not high enough Coal combustion techniques signifcantly infuence the for mullite formation; thus, no difraction peaks of mullite morphology, particle shape, and particle size distribution of can be observed. Instead, hematite, phengite and anhydrite fy ash. Figure 1 presents the scanning electron microscopy gehlenite might be present in the XRD patterns of FBC fy image of fy ash from four coal-fred thermal power plants. ash [4, 55, 57]. Figure 3 presents the XRD patterns of four Due to high-temperature (>1400 C) combustion, the fux types of fy ash. PCC fy ash shows only two crystal phases, agents in the PCC fy ash melted, stuck to unmelted particles, quartz and mullite, while FBC fy ash shows three crystalline and rounded under surface tension, forming spherical phases, including quartz, phengite, and hematite. particles upon cooling. On the other hand, FBC fy ash In addition to oxide solids, fy ash may contain signif- particles have irregular shapes as they were created from a icant amounts of unburnt carbon (UC), a result of in- low-temperature (<900 C) combustion process and thus did complete combustion. Te UC content depends on the not have sufcient liquid phase to form spherical particles. combustion technology, operating conditions, and coal type. Fly ash is a collection of fne particles that have harmful It is assessed by the loss-on-ignition (LOI) test [58]. Termal efects on human health and ecology. Signifcantly, the wind analysis of fy ash confrms the presence of UC with a large can blow fne fy ash particles long distances, making fy ash a exothermic peak incorporated with weight loss. Te starting global health and environmental concern [41, 42]. Most coal point of UC combustion in TG-DTA analysis varies with the fy ash has particle sizes under 100 micrometers [43–45]. type of coal fy ash [55, 59, 60]. Typically, the amount of UC Figure 2 shows the particle size distribution curves of four in fy ash is higher than that in bottom ash [61]. types of fy ash. Tey present the frequency distribution (q in Fly ash UC is an inexpensive source of activated carbon %) of particles within a specifc size range. Although the for adsorbing harmful components of fue gas such as NO particle size distribution curves are diferent and the com- [62], mercury vapor [63, 64], and organic substances [65]. bustion methods show no trend in these curves, they all have However, UC is a factor hindering the use of fy ash in many one thing in common: 90% of each sample (D90) is smaller construction applications. From the concrete perspective, than 100 micrometers and the median particle size (D50) is UC adsorbs air-entraining admixture, reducing the physical in the range of 12.4–17.6 micrometers. properties of concrete [66]. As specifed by ASTM C618 - 19 Te chemical composition of fy ash is quite complicated, standard, the LOI content of fy ash used as pozzolanic depending on the coal origin and the combustion technique. additives in concrete should be under 6% with some tol- By weight percentage, dominant in fy ash is SiO , followed 2 erance to the Class F fy ash [12]. by Al O , and other oxides such as Fe O , CaO, K O, Na O, 2 3 2 3 2 2 Te leaching test performed on fy ash by the EN and TiO (Table 1). Te introduction of limestone to the FBC 2 12457–4 test method shows that the heavy metal concen- boilers adds CaO to the solid combustion residues. Fly ash is trations in the leachate are below the inert waste limits also a source of heavy metals, including but not limited to V, specifed by the European Landfll Directive [67]. However, Cr, Mn, Co, Ni, As, and Hg [46, 48, 49]. In addition, there are the long-term leaching test indicates a potential risk to soil distinct diferences between the chemical composition of and groundwater [68]. As has been demonstrated in various diferent size fractions of fy ash from a boiler. Signifcantly studies, it is possible to immobilize heavy metals of solid higher concentrations of total carbon and heavy metals are wastes from wastewater treatment plants [4, 69], red sludge found in smaller particles. Slightly higher concentrations of from bauxite processing plants [70], and blast furnace slag major elements, Si, Al, Fe, Ca, K Na, Mg, Mn, and Ti, are [71–73] in the structure of glass [74], glass ceramics [75], and present in coarser ones [46, 48]. Fortunately, most heavy ceramic materials [76]. Likewise, introducing fy ash to metals are locked in the glass phase of fy ash [50]. However, ceramic production is a potential solution to lock the heavy the ability to release heavy metals and rare Earth elements metals of fy ash in the ceramic network. from fy ash in landflls is always a community concern. Te Due to high SiO and Al O contents, fy ash is suitable 2 2 3 allowable limits for the disposal of coal combustion residuals as a precursor for aluminosilicate crystalline phases, such as vary between countries and regions but may be tightened in mullite [77, 78] and cordierite [79–82]. In addition, the high the future [51, 52]. CaO content in certain types of fy ash, especially those from Te mineral composition of fy ash depends on the coal FBC boilers using limestone to absorb SO , makes them type and combustion techniques. X-ray difraction (XRD) suitable as a raw material for glass ceramics that contain analysis of a series of fy ash showed a signifcant variation in calcium silicate and calcium aluminosilicate crystalline the glass phase content, from 45 to 80% [53]. Besides the phases [83–86]. Furthermore, due to its high glass phase glass phase, quartz is the most popular crystalline phase. Its content, fy ash is also considered a fuxing agent that difraction peaks stand out in the XRD patterns of all types of promotes sintering in ceramics. coal fy ash. Since PCC boilers operate at temperatures above 1400 C, high enough for mullite crystallization, its fy ash 4. Firing-Associated Recycling of Fly Ash often contains mullite (3Al O .2SiO ) [54, 55]. Te mineral 2 3 2 composition of mullite is heterogeneous due to the sub- 4.1. Bricks. Fly ash can replace up to 80% of clay in clay stitution of impurities at the aluminum sites, forming solid bricks, but some adjustments should be made. Te presence solutions. XRD and nuclear magnetic resonance analysis on of a large amount of fy ash signifcantly reduces the raw mix a bituminous coal fy ash showed a mullite average chemical plasticity index. Terefore, it is necessary to add plastic 4 Journal of Analytical Methods in Chemistry (a) (b) (c) (d) Figure 1: Scanning electron microscopy images of fy ash: (a) Uong Bi PCC, (b) Ninh Binh PCC, (c) Dong Trieu FBC, and (b) Mong Duong I FBC. additives to facilitate extrusion. In addition, the porous threshold limits of EPA Victoria (Australia) for industrial structure of fy ash particles impedes the sintering process, solid waste. A study by Leiva et al. also shows that the increasing the porosity of the sintered product even at low leaching contents are below the limits of the European degrees of replacement [87]. Hence, it may be necessary to Landfll Directive for granular waste, the Italian national grind the fy ash to break down its porous structure and regulation for the reuse of waste in construction material, increase the contact surface area to speed up the sintering and the Dutch Soil Quality Decree for bound or shaped process [37]. Along with the high porosity, the high re- materials [47]. fractoriness of fy ash requires an increase of the fring temperature by 50 to 100 C so that the physical properties of 4.2. Ceramic Tiles. Fly ash with high fux the sintered products meet those of conventional clay bricks. Most reports indicate that when using large amounts of fy (Fe O + Na O + K O) contents can be used to replace 2 3 2 2 feldspar in ceramic tile production. Olgun et al. were able to ash, the fring temperature is 1050 C or higher [37, 88]. A test at the industrial scale reveals a number of obstacles that improve the modulus of rupture of wall tiles by replacing potassium feldspar in the raw mix with fy ash and borax should be overcome: brick swelling and deformation due to local melting and the occurrence of black core and cracks solid waste without having to change the sintering tem- perature [90]. As the porous structure of fy ash hinders [89]. It is possible to manufacture bricks with 100% of the sintering, resulting in high porosity, it is mainly used as a raw material for wall tiles, equivalent to Group BIII as classifed solid material being fy ash. In this case, shaping requires a temporary binder because most fy ash lacks adhesive ca- by EN 14411. Tis type of ceramic tile is not subject to the loading force in use but requires high water absorption for pacity. Kayali reported that although the apparent density of good mortar adhesion, allowing higher water absorption and the fred product is 28% lower than that of conventional clay bricks, its compressive strength is 24% higher, and brick-to- lower fexural strength than those of the foor tiles [91]. Similar to that observed in clay bricks, the disruption of mortar bond strength is 44% higher than those of the best local clay bricks [8]. porous fy ash particles also improves the sintering of the ceramic tiles [92]. When high alumina fy ash is used and a It is noteworthy that heavy metals are immobilized in the ceramic structure of the brick. Sutcu et al. [88] showed that foor tile is the desired product, the fring temperature must be increased to achieve the required physical properties. In the release of heavy metals, including Cr, Mn, Ni, Cu, Zn, As, Cd, Ba, Hg, and Pb, was substantially lower than the particular, the sintering process becomes more difcult when the Al O content is up to 40%, equivalent to freclay maximum allowable limits regulated by U.S. Environmental 2 3 Protection Agency (EPA) for hazardous solid waste and the refractory bricks. Wang et al. showed that for producing Journal of Analytical Methods in Chemistry 5 Uong Bi PCC Ninh Binh PPC Dong Trieu FBC Mong Duong I FBC 0.1 1 10 100 1000 Diameter (μm) Figure 2: Fly ash particle size distribution. Table 1: Chemical composition of fy ash. Chemical Ninh Binh, Mong Duong I, Seyitomer, Tennessee, US XianYang, Bokaro, Bokaro, Spain [47] a a b,d c,d composition Vietnam Vietnam Turkey [7] [15] China [33] India [46] India [46] SiO (wt.%) 45.20 52.16 57.21 49.54 54.39 39.57 51.94 45.02 Al O 2 3 19.99 23.65 20.39 19.11 23.21 22.13 26.41 25.18 (wt.%) Fe O 2 3 6.86 5.97 10.89 14.79 11.50 3.21 5.21 16.52 (wt.%) CaO (wt.%) 1.52 1.94 2.75 6.23 4.77 0.42 0.49 8.53 MgO (wt.%) 1.29 1.09 4.96 0.42 1.73 0.19 0.27 0.97 K O (wt.%) 3.13 3.30 1.36 — 1.32 0.93 0.95 2.14 Na O (wt.%) 0.32 0.19 0.40 — 0.37 0.13 0.16 0.43 TiO (wt.%) 0.57 0.91 0.81 — — 2.08 2.09 — SO — — — 0.34 — 1.45 0.50 — LOI (wt.%) 20.15 9.46 0.94 2.43 13.38 29.8 9.67 0.7 Cd (ppm) — — — — — — — 0.9 Pb (ppm) — — 79.0 — — 31.06 63.42 58 Zn (ppm) — — 112.6 — — 47.2 76.6 309 Cu (ppm) — — 98.8 — — 73.8 97.7 - Cr (ppm) — — 454.5 — — 140.7 137.4 149 Ni (ppm) — — 1975.9 — — 77.5 65.7 74 Mn (ppm) — — 790.4 — — 117.8 268.4 - a b c Present work, by chemical analysis. Fraction retained by Sieve No. 150 BS (+104 μm). Fraction passed through by Sieve No. 300 B.S and retained by Sieve No 350 BS (−53 μm to + 45 μm). Incomplete list of the analyzed heavy metals and rear Earth elements from the cited sources. q (%) q (%) q (%) q (%) 6 Journal of Analytical Methods in Chemistry Q Q Q Q Q H Q Mong Duong I FBC Dong Trieu FBC Cam Pha H M Uong Bi PCC 10 20 30 40 50 60 2-theta (deg.) Q: Quartz P: Phengite M: Mullite H: Hematite Figure 3: Te X-ray difraction patterns of fy ash samples. ceramic tiles with water absorption below 0.5% from a raw concentrations in the leachate of ceramic tiles. However, one mix with 70% high alumina fy ash, the sintering temper- can infer from the studies on bricks that ceramic tiles are also ature is as high as 1300 C [93]. Solid wastes with high fux capable of immobilizing hazardous elements, even better contents, such as glass waste [94, 95], boron waste [90, 96], than bricks because of their denser structure formed by or lithium mine tailings, are viable additives that help lower higher sintering temperatures. the sintering temperature [97]. Wang et al. used high alumina content fy ash (40% 4.3. Insulating Materials and Lightweight Concrete Aggregates. Al O ) and waste glass to make insulating ceramic tiles, 2 3 consisting of a layer of foam and a layer of dense material. Insulating materials and lightweight concrete aggregates feature porous structures constructed by closed pores. Te Te raw mix of the foam layer consists of waste glass (50%), sintering of these materials requires a simultaneous gen- fy ash (30%), clay (15%), and feldspar (5%). Te raw mix of the dense layer consists of fy ash (60–80%), clay (15%), and eration of gas and molten phases with sufcient quantity and viscosity. Te molten phase entraps the gas, forming closed quartz (5–25%). With single-loading pressing and calcina- tion at 1200 C, the foam layer performs insulation capacity pores upon cooling. Fly ash is an excellent candidate for this application due to its UC and high glass phase contents. Te with an average pore diameter of 300–500 micrometers. In addition, the bi-layer ceramic shows a fexural strength of UC oxidizes at elevated temperatures releasing carbon di- oxide gas while the glass phase is ready to melt. However, if 31.5 MPa. Tis way, over 70 weight percent of the tile originates from industrial waste [94]. fy ash is the only raw material, the fring temperature should be up to 1300–1400 C for bloating [100]. Furthermore, UC Most fy ash has a Fe O content of 4–10% (Table 1). Te 2 3 presence of a remarkably high Fe O content gives a clay might be low in certain fy ash sources (Table 1). Terefore, 2 3 fuxes and gas-forming agents are required for (1) generating brick color to fy ash-based ceramic bodies [8]. On the other gases, (2) reducing the fring temperature, and (3) adjusting hand, the combination of Fe O and UC causes the black 2 3 core in bricks [98]. Terefore, if masking the ceramic body the quantity and viscosity of the liquid phase for entrapping the generated gases. Te fuxes of interest include waste glass color is desired, an engobe glaze with high opacity and whiteness should be applied on the surface of the tiles before [101, 102], limestone [103, 104], and synthetic chemicals such as sodium salts [105, 106]. Some fuxes, including the decoration is fnished. Although a test method for determining lead and cad- limestone and soda, also play the role of gas-forming agents. Since most fy ash is not self-adhesive, a temporary mium leaching from ceramic tiles is available [99], there is no harmonized requirement for hazardous element binder is required to form the green pellets. On a laboratory Intensity (a.u.) Journal of Analytical Methods in Chemistry 7 scale, organic binders such as polyvinyl alcohol have been et al. fabricated porous materials from coal mine waste tested [100]. On a larger scale, clay is a popular binder due to kaolin and spherical hollow fy ash, using bentonite as the its availability and low cost [103, 106]. Besides, bentonite binder and calcium iodate (Ca(IO ) .6H O) as a focculant. 3 2 2 [102, 104] and ordinary Portland cement [104] have been After fring at 1550 C, the resulting product has a porosity of used. 44.73–46.12%, with 99% of the porosity being closed pores, and mullite is the main crystalline phase. Te freeze-gel casting/polymer sponge technique can produce a porous 4.4. Mullite and Cordierite Ceramics. Fly ash is also a raw mullite ceramic with a porosity of 66.1% and an average material for the production of mullite ceramics due to its high compressive strength of 45 MPa [112]. SiO and Al O contents. Low alumina fy ash often requires an Fly ash is also suitable as a raw material for cordierite 2 2 3 alumina augmenting agent such as aluminum oxide [107], ceramics, a material well known for its low LTEC. Cordierite bauxite [77, 108], or high alumina industrial wastes [78, 109]. crystallizes in the orthorhombic system. Its dimorphism is Unlike FBC fy ash, mullite is usually available in PPC fy ash. hexagonal indialite which is more refractory than cordierite Te presence of mullite makes the mullite crystallization in PCC [113–115]. Although the IMA formula of cordierite is fy ash-based ceramics easier than in those using FBC fy ash. 2MgO.2Al O .5SiO , it appears as a solid solution in practice 2 3 2 Although fuxes are available in fy ash and start forming [116, 117]. Tanks to its low LTEC, cordierite ceramic is a liquid phase at relatively low temperatures, fy ash-based highly resistant to thermal shock. Tis material also pos- mullite ceramics are often sintered at high temperatures, sesses high chemical resistance and a low dielectric constant. usually above 1400 C, because mullite is a refractory phase. Te synthesis of cordierite from fy ash requires MgO Making dense mullite ceramics from PCC fy ash and supplementing materials such as talc [118], magnesia [119], bauxite, Dong et al. showed that the solid-state reaction magnesite [120], or dolomite [121]. Unlike mullite, the between cristobalite and corundum occurs at temperatures sintering temperature of cordierite ceramics is usually below ° ° below 1300 C, followed by the dissolution of the corundum 1200 C. In addition to indialite, fy ash-based cordierite into the liquid phase at higher temperatures where sec- ceramics may contain mullite, cristobalite, periclase, and ondary crystallization occurs. Te formation and recrys- spinel [118, 120]. Porous cordierite ceramics from fy ash can tallization of mullite lead to volume expansion which is be used for microfltration membranes [82], catalytic sub- slightly dominant over the shrinkage of the sintering pro- strates [118], and membrane supports [121]. cess. At 1600 C, the material achieves a relative density of 93.94% with spherical pores and fracture strength of 186.19 MPa [77]. Jung et al. combined aluminum oxide with 4.5. Foam Glass and Glass Ceramics. Tanks to its high PCC fy ash, from which UC was removed, to produce silicate glass content, fy ash can be used as a raw material for mullite ceramics at temperatures of 1400, 1500, and 1600 C. glass and glass-ceramic synthesis. Erol et al. produce glass by Although the pellet is cold isostatic pressing, the density of melting fy ash at 1500 C, glass ceramics by annealing the the fred product was relatively low (63%). Tis low density is obtained glass at 1150 C, and ceramics by fring green pellets due to the exaggerated grain growth of needle-shaped at 1200 C. Te only phase in the glass products is amor- mullite crystals incorporated with voids formation [107]. phous, the crystalline phase in the glass-ceramic products is 3+ Te introduction of the supplements alters the sintering augite (Ca(Mg, Fe , Al) (Si, Al) O ), and in the ceramic is 2 6 mechanism either by preventing excessive grain growth or quartz, mullite, and enstatite ((Mg, Fe)SiO ) [34]. However, by creating new phases, thereby improving the physical making glass and glass ceramics products from fy ash can be properties of the sintered products. Te presence of 3Y-PSZ difcult due to high SiO and Al O (network formers) 2 2 3 inhibits the crystal growth of mullite, leading to an improved contents and insufcient fuxes (network modifers). fracture strength [107]. Magnesia efectively promotes sin- Terefore, it is necessary to add fux ingredients to lower the tering, signifcantly above 1450 C. It slightly reduces the melting temperature. linear thermal expansion coefcient (LTEC) at 1300 C by One of the most attentive applications of fy ash-based forming low thermal expansion α-cordierite. However, it glass is glass foam, which is considered a low-cost insulation slightly increases the LTEC above 1400 C due to the for- material in construction. Besides fy ash, glass foam raw mixes mation of high expansion corundum and the spinel often have fux and foaming agents. Most of them are in- (MgAl O ) [108]. SiC enables the growth of mullite needle- dustrial wastes. Te commonly used fux is waste glass. It is 2 4 shaped crystals out of the saturated glass phase, resulting in used in large quantities comparable to fy ash [33, 122]. Al- better bonding of the crystals, reducing the true porosity but ternatively, borax [122] and soda [123] can be added to the raw increasing the closed-pore porosity. As a result, thermal mix as network former and modifer, respectively. Foaming conductivity and cold crushing strength are improved [110]. agents can be obtained from a variety of sources, including TiO hinders the sintering process at low temperatures but calcium carbonate [122], soda [123], and SiC [33, 124]. For promotes sintering above 1300 C. Tis phenomenon is instance, Bai et al. produced glass foam by melting a raw mix of useful for the unsaturated sintering of porous mullite ce- fy ash, waste glass, and SiC waste at 950 C. Te resulting ramic membrane supports [111]. product expanded 5.81 times compared to the green body [33]. Te porous structure of fy ash particles and the re- Another fascinating application of fy ash-based glass is crystallization of mullite favor the fabrication of porous glass ceramics because of their high mechanical strength and mullite ceramics for insulation or fltration purposes. Chen thermal shock resistance. Te crystalline phase in fy ash- 8 Journal of Analytical Methods in Chemistry based glass ceramics is controlled by adjusting the com- 6. Conclusion position of the raw mix. Te two glass ceramic systems of Coal fy ash is a global concern due to its small particle size great interest are SiO2-Al2O3-CaO [125, 126] and SiO - and heavy metal contents with increasing emissions. Faced Al O -MgO [79, 127]. When a high amount of Fe O is 2 3 2 3 with concerns about wind-blowing dust and heavy metal available in fy ash, the system SiO -Al O -Fe O -CaO is of 2 2 3 2 3 leakage from fy ash landflls and storage sites, eforts to use interest [128]. Depending on the chemical composition of fy ash for safe and value-added applications are underway. the mix, the crystalline phases in the SiO -Al O -CaO glass- 2 2 3 Notable among them are fring-associated measures, in- ceramics include diopside (Ca(Mg, Al) (Si, Al) O ) [125], 2 6 cluding the production of ceramics, lightweight concrete augite (Ca(Mg, Fe)Si O ) [34, 125], and wollastonite 2 6 aggregates, glass, and glass ceramics. Tere is almost no (CaSiO ) [129]. SiO -Al O -MgO glass ceramics are of the 3 2 2 3 restriction for fy ash in these applications. Most notably, the most interest because they contain cordierite, a mineral with recycling of fy ash and other industrial solid wastes can be low LTEC, and are suitable for thermal shock applications combined in these applications without violating regulations [79, 127]. on heavy metals released during use. Besides, the unburnt carbon content can alleviate the heat required for fring. Te 4.6. Portland Cement Clinker. In cement production, fy ash fring temperature, product phase composition, and physical often serves as a pozzolanic additive, but high UC fy ash is properties of the products can be controlled by supple- not suitable for this application. Another possibility of menting agents. However, the sulfur content might be high using fy ash in cement production is to replace clay in the in certain types of fy ash and go into the fue gas as sulfur raw mixes of Portland cement clinker [130–133], belite dioxide. Tis problem can be solved in cement kilns but cement clinker [134], and belite-sulfoaluminate cement should be addressed in other applications. clinker [135]. Tis perspective has been investigated from the laboratory to the industrial scale. Laboratory studies Data Availability showed that the use of fy ash reduced the sintering All data generated and analyzed to support the fndings of temperature of Portland cement clinker [130, 131]. this study are available within the article. Commercial demonstration on high carbon fy ash showed that the obtained fy ash-based Portland cement Conflicts of Interest performs a higher compressive strength than the normal ones et al.l ages, despite a lower fneness [132]. Te use of Te authors declare that they have no conficts of interest. high carbon high fy ash has the additional beneft of fuel- saving [133]. Authors’ Contributions Unlike ceramic fring, SO formed in the fring process of clinker can be reabsorbed by calcium oxide. Te resulting Conceptualization and supervision were done by Vu Ti calcium sulfate helps reduce the gypsum amount required Ngoc Minh. Vu Ti Ngoc Minh reviewed the article. Data for cement setting control [136]. Terefore, using fy ash as accuracy was tested by Pham Hung Vuong. Pham Hung raw material for cement clinker brings valuable economic, Vuong reviewed the article. Conceptualization and data technical, and environmental benefts. accuracy were done by Vu Hoang Tung. Data analysis was performed by Cao To Tung. Cao To Tung reviewed the 5. Challenges and Opportunities article. Nguyen Ti Hong Phuong performed data analysis. Although it is demonstrated that fy ash-based glass and Acknowledgments ceramic products can immobilize the heavy metals and rare elements of fy ash in their oxide networks, they are not Tis work was fnancially supported by the Vietnam Min- suitable as food and drug containers. Tis restriction is istry of Training and Education under grant number B2020- mainly due to the stringent regulations applied to these BKA-17. Te authors wish to thank Mr. Mai Van Vo and Ms. products [137–139]. On the other hand, SO emissions x Bui Ti Tu Ha for their valuable technical support. from burning fy ash are also of concern because the SO content in certain types of fy ash may be high. 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