A Review on the Utilization and Environmental Concerns of Coal Fly Ash

ABSTRACT


INTRODUCTION
The popular source of electric power generation around the world today is coal utilized in pulverized coal combustion and fluid bed combustible conventional plants(S. H. Lee, Lee, Jeong, & Lee, 2019;Miller, 2010). The two different combustion methods operate at different temperatures and therefore each method produces different fly ash with different characteristics (Erol, Küçükbayrak, & Ersoy-Mericboyu, 2008;L. Li, Wang, & Zhu, 2006;Molina & Poole, 2004). The temperature range in the coal fluid bed is 8500C-9000C while the temperature in pulverized combustion is between 14000C-15000C. At temperatures greater than 14000C (>14000C), minerals such as the aluminosilicate present in coal disintegrate or decompose (Mishra & Baliarsingh, 2008;Pavlish et al., 2003;Rios Reyes, 2008). The disintegrated mineral particles form hollow or spherical objects called cenospheres as a result of rapid cooling that takes place during the postcombustion phase (Kim, 2002;Pundienė, Korjakins, Pranckevičienė, & Kligys, 2018). High melting point minerals sometimes remain unaltered. The physical and chemical composition of fly ash is significantly influenced by the type of coal used and the variation in combustion temperatures (Adriano, Page, Elseewi, Chang, & Straughan, 1980;Pedersen, Jensen, Skjøth-Rasmussen, & Dam-Johansen, 2008;Ratafia-Brown, 1994). Both two types of combustion processes contain Al, SiO 2 , Fe, and Ca which are the four most constituents in fly ash (Cheng & Chen, 2003;Criado, Fernández-Jiménez, De La Torre, Aranda, & Palomo, 2007). The fly ash produced from coal fluidized beds (CBD) has no mullite and glass balls. It contains high quantities of unburnt carbon, calcium carbonate, and calcium sulfate and it forms irregular objects. Unburned carbon exists in both types of fly ash (Batra, Urbonaite, & Svensson, 2008;Maroto-Valer, Taulbee, & Hower, 2001). Fly ash is among the popular global raw material and one of its most important utilization is in the fabrication of composites (Gollakota, Volli, & Shu, 2019). In addition to the current global use of fly ash, further research and development are required to enhance the management of fly ash and bring out new techniques to control the end products that result from burning coal. Harmful elements and compounds such as aromatic compounds, carbon, silica, fine particulate matter, etc., are aggregated in high concentrations during combustion (Schwarze et al., 2006;Valavanidis, Fiotakis, & Vlachogianni, 2008). Thermal power plants produce enormous inorganic particles and emit combustible gases into the atmosphere which negatively affects the health of residents around the thermal plants (Rahman, Farrok, & Haque, 2022). The thermal industries have no use for the ash generated and therefore regard it as a solid waste product with the potential to negatively impact the environment if not properly regulated (Foday Jr et al., 2017). A small amount ( ≈20%) of the global fly ash https://journals.e-palli.com/home/index.php/ajcp Am. J. Chem. Pharm. 2(2) 41-52, 2023 produced annually, is used for many purposes and the rest reportedly contributes to the existing global pollution trend. Characterizations using SEM, EDS, XRD, FTIR, etc, to determine the composition of fly ash have been studied. Several past works on the two types of fly ash were centered on their properties, morphology, and chemical composition. In this study, the morphology and chemical information of the fly ash samples is determined by SEM and EDS. The XRD reveals information about the crystalline size and amorphous phase of the samples. The in-situ functional groups (Si-O-Si, C-O, Al-O, etc,) in fly ash are accounted for by the Fourier transform infrared spectroscopy (FTIR). These functional groups are the typical determinants for the use of fly ash as raw material for the synthesis of composites. This experiment, therefore, studies the characterization of purified fly ash from the Xi'an Linyuan silica limited. The major problem encountered in the characterization of fly ash is the amorphous phase of the sample which is normally not captured by the XRD. A more adequate instrument is therefore required for the characterization of fly ash to identify its glass phase and interpret chemical composition. EDS is the most common effective tool used for this purpose but the contrasting characteristics of fly ash present a large scattered result that makes the analysis very difficult. In recent times, new EDS technology or detectors have been invented with the capability to collect the spectrums of all pixels in not more than an hour. The spectra collected are further processed into maps that show the elemental intensities used to quantify the chemical compositions of the elements present.

LITERATURE REVIEW Process and end products of coal combustion
Coal constitutes inorganic minerals released as waste products (fly ash, bottom ash, and slag) during the burning of coal. The greater part of the waste products in the combustion plant is made of fly ash. Fly ash is made of fine light particles and has a density of about 2.5g/cm 3, bottom ash is composed of coarse particles while slag is the melted phase of bottom ash (Bourtsalas, 2015;Bunge, 2015). The ash generated in the combustion plant is made of a large mass of oxides (silica, alumina, titanium oxide, magnesium oxide, calcium oxide, sulfoxide, etc,) and heavy metals (zinc, lead, copper, nickel, cobalt, cadmium, etc,) (Foday Jr, Bo, & Xu, 2021;Halmann, 1995;Oves, Khan, Zaidi, & Ahmad, 2012). Fly ash contains some quantities of spherical matter called cenospheres formed by the decomposed minerals in the condensation or cooling stage (Blissett & Rowson, 2012;Fenelonov, Mel'gunov, & Parmon, 2010;Oves et al., 2012;Vassilev, Baxter, Andersen, & Vassileva, 2013). Fly ash is formed due to the incomplete combustion of the mineral components in coal which subsequently transform into some crystalline phases called quartz (silica), magnetite, aluminosilicate glass (mullite), spinel, etc. It is confirmed that fly ash is made of over 80% of silica (SiO 2 ) (Criado et al., 2007;Joshi & Lohita, 1997;Vassilev & Vassileva, 1996). The glass phase in fly ash is formed from low-grade bituminous coals which constitute lesser quantities of silica and a higher amount of magnetite compared to high-grade bituminous coal which composes high silica content. The process leading to the formation of fly ash is explained in figure 1 below:

Figure 1: A modified schematic diagram for the formation of fly ash
The process begins with vaporizable components which nucleate to produce new crystalline objects, form char and release heavy metals by vaporization.

Reactions in coal combustion
The carbon in coal undergoes the following chemical reaction with oxygen.
https://journals.e-palli.com/home/index.php/ajcp Am. J. Chem. Pharm. 2(2) 41-52, 2023 The reaction is exothermic when the enthalpy ∆H change is negative (i.e. heat is given out during the reaction) while a positive enthalpy ∆H means the reaction is endothermic (i.e. heat is absorbed during the reaction). The first two reactions are the main combustion reactions according to their products. CO 2 and CO are the first combustion products and their ratios increase with an increase in temperature and a decrease in pressure. The whole combustion process leading to the formation of a coal boiler takes about two seconds.

Mathematical models for fly ash deposition in a boiler tube
Mathematical models are employed to determine the activity of ash precipitation and to ameliorate the lapses in industrial design (Fan, Zha, Sun, & Cen, 2001). The prediction of ash deposition has been done by a good number of researchers using experimental notations but it becomes misleading when taking into consideration other operating factors (Fan et al., 2001;Zeng, Zheng, Zhou, Fang, & Lou, 2016). The deposition of ash in boilers has been adequately studied with the application of numerical procedures. Fly ash data obtained from a computer control scanning electron microscope has been used to develop a fly ash prediction model (F. Lee & Lockwood, 1999). The temperature, porosity, and rate of deposition were simulated by Galen et al (Richards, Slater, & Harb, 1993).

Particle Movement Expressions
Newton's equation of motion below is used to account for the particle movement in a pulverized boiler.
The drag force F d is given by where μg is the gas viscosity and f is the drag force coefficient that is used to revise the Stokes expression and assuming that f=1. Waldmann equation below is used to determine the thermophoretic force (Waldmann & Schmitt, 1966) Where K tr is the thermal conductivity of the gas and ∆T is the temperature gradient. The mean speed c of gas molecules is given by

Particle Collision Forces
The particle equation is used to compute the collision velocities of fly ash deposition in a boiler. To ensure efficient and smooth velocity collision, the particle in motion must be spherically rigid, the interaction forces and infinite forces should be impulsive and negligible respectively, and the particle movement should be centered and two-dimensional. The mathematical expression for a particle is given below (Hoomans, Kuipers, Briels, & van Swaaij, 1996).
where, v i.0 and v i,1 is the pre-collisional and post-collisional velocities of particle i, and v j,0 and v j,1 are the pre-collisional and post-collisional velocities of particle j. In the absence of particle-particle collision, we can also find the sample position using the single-particle equation at a time. The method of Monte Carlo is exclusively used to compute the particle-particle collision. If particle i collides with particle j in the time step, the post-collision velocities of particles are accounted for by taking into consideration the dynamics of the collisions. The velocities after particle collision will now substitute the initial particle velocities but the particles remain fixed in their fixed positions. The particle i in a time step Δt is determined by the probability of the collision below (Bird, 1994;Tsuji, Tanaka, & Yonemura, 1998): where N is the number of simulated particles in the cell, n is the local particle number, d is the particle diameter, go is the radial distribution function, v ij v is the relative velocity between particle i and j, and ∆t is the time step. The collision pairs searching is the key problem in the calculation of particle-particle collision.

Ash Deposition Model
During the process of ash deposition, the large ash particle causes an impact in the boiler tube and rebounds while the smaller ones can stick onto the tube with high adhesive energies(G. Li, Li, Huang, & Yao, 2015). The formation of the first layer improves the surface sticking capability in the tub. The critical velocity model is employed to ascertain if a particle motion sticks or rebounds in the boiler tube. The critical velocity equation is written as: Where, W is the adhesion energy and E is Young's modulus (S. Li, Marshall, Liu, & Yao, 2011;. According to the simulation of the critical velocity ( Vcritical ) expression above, the impact of the particle velocity is compared with the critical velocity. When the critical velocity of a particle is larger than the velocity impact, the particle clings to the surface.

MATERIALS AND METHOD
Five samples of fly ash were measured on an electronic weighing scale. Each sample weighs 4g. Four of these samples were heated at 500 0 C, 600 0 C, 700 0 C, and 800 0 C for 3 hours in a muffle furnace and one remain unheated. The samples were then taken for SEM, EDS, XRD, and FTIR characterization. The aim was to determine the elemental characteristics of fly ash that make it a suitable raw material to synthesize refractory composites.

RESULT AND DISCUSSION SEM Result
This technique is utilized by material scientists to determine and interprete the microstructure of materials. The sample electrons are bombarded to produce a wide range of emissions (such as X-rays, visible photons, auger electrons, secondary electrons, backscattered electrons, etc). The SEM result of the fly ash specimens is discussed as follows:  All four sample images show un-interconnected irregular oval structured layers in both figures A and B. Few geometrically shaped structures are observed in sample 3. The agglomerated oval glassy and geometrically shaped structures are typical of the presence of silica crystalline structures in the sample materials. The balloon-shaped crystals are indicative of the presence of cenospheres in the fly ash samples. There are significant pore sizes in the samples and are observed to be highly visible in specimens 1, 2, and 4 but less in specimen 3. This shows that there might be other materials present in the samples but in less or insignificant quantities. All four samples heated at various temperatures show that quartz or silica is the dominant crystalline phase present in the fly ash sample. The samples were heated at temperatures that will lead to the elimination of carbon contents present in the fly ash with no chemical reaction taking place. According to the micrographs, the oval crystalline structures become more pronounced as the temperature increases from 500 0 C, 600 0 C, 700 0 C, and 800 0 C. Conversely, the dark fields observed in the micrographs according to the morphology of the sample become lesser as the temperature is increased from sample one to four. This can be attributed to the carbon content which decreases as more temperature is applied to the fly ash. Figure 4 shows the SEM micrographs of the unheated fly ash specimen with image sizes scanned at 200 um, 30 um, and 20 um pixels. The oval crystals are the same as those of the heated samples shown in figure 3 but are visibly observed to be far apart from each other compared to those seen in figure 3A above. This probably implies the presence of carbon content and other minor constituents that occupies the spaces between the crystals.

EDS Result
This is a powerful analytical detector or tool used to determine the chemical elements present in a sample and estimate their relative abundance. It relies on the interaction of some source of X-ray excitation and a sample. The capability of this tool is due to the fact or principle that each element has its unique structure that allows several peaks to display on its electromagnetic emission spectrum (which is the main principle of the electromagnetic spectrum). In the emission of X-ray characteristics from a sample, a beam of electrons is focused on the sample under investigation. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell and forming a hole in the initial position of the electron. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lowerenergy shell may be released in the form of an X-ray. The EDS is then used to measure the number and energy of the X-rays emitted from the sample. The detector or tool allows the elemental composition of the sample to be measured since the energies of the X-ray are characteristic of the atomic structure of the emitting element. The EDS shown in figures 5 and 6 confirmed the percentage weight and elemental composition of both heated and unheated fly ash samples. According to the EDS result, oxygen and silicon are the predominating elements present in all five prepared samples. The percentage composition of oxygen is greater than that of silicon in all five samples. It is also observed from the elemental tables of the EDS figures that the weight percentage of silicon is higher in the first, third, and unheated samples than those of oxygen atoms but lesser in the second and fourth samples. The results and analysis of this work show that fly ash used is silica pure and therefore a recommendable source for the fabrication of refractory composite materials.

Cenospheres
According to the SEM and EDS results of this research, the hollow and spherical-shaped objects observed in all five samples depict the presence of cenospheres. Cenospheres are one of the end products in coal combustion and are significant components for the synthesis of refractory composites. They are formed as a result of the decomposition of the mineral matter in coal at high temperatures >1400 0 C. The disintegrated or decomposed mineral matter form into spheres known as cenospheres during the post-combustion phase or cooling process. The decomposition of calcium carbonate and other compounds below 1000 0 C produces gases that blow up the minerals. The gases emitted during coal combustion inflate the decomposed minerals to form cenospheres. These objects are largely made of mullite, quartz (silica), aluminosilicate glass, calcite, sulfates, etc. Inside the balloon-shaped structure of the cenospheres are gases while the outer membrane or layer is made of aluminosilicate, cristobalite, quartz, mullite, etc. Cenospheres are discovered to be highly porous and can therefore effectively absorb water.  The cenospheres discovered from the fly ash samples in this experimental research can be used for electronic and radar purposes due to their metallic and magnetic properties. Cenospheres are useful to refractory industrial engineers due to their low thermal conductivities and high heat resistance. They also have properties that are suitable to make lightweight composite materials used in aerospace engineering.

FTIR Result
This technique is employed to study the vibrations of atoms in a molecule or compounds of a particular sample. The fractions of absorbed incident radiation at a particular energy are determined by passing Infrared (IR) radiation through the specimen. The FTIR results and analysis of the prepared fly ash specimens are discussed below.
https://journals.e-palli.com/home/index.php/ajcp Am. J. Chem. Pharm. 2(2) 41-52, 2023 Heated Samples The figure below shows the FTIR spectra obtained from the characterization of fly ash samples heated at 500 0 C, 600 0 C, 700 0 C, and 800 0 C. According to the experimental result of the sample heated at 500 0 C, a strong appearance of O=C=O bending of carbon dioxide compound is observed at a wavenumber of 2349cm -1 . Further, the wavenumber 1083cm -1 depicts the appearance of Si-O-Si stretching vibrations and strong absorption peaks while the 799cm-1 wavenumber shows Si-O symmetric stretching of quartz. Analysis of the FTIR experimental data for the sample heated at 600 0 C also depicts the appearance of O=C=O at 2349cm -1 which indicates a strong vibrational stretching of the carbon dioxide compound. The wavenumbers 1090cm -1 and 807cm -1 indicate Si-O-Si and Si-O stretching vibrations respectively. In sample three, the wavenumber 2363cm -1 from FTIR results shows O=C=O bending but does not fall at 2349cm -1 which is the strong absorption band for O=C=O. Therefore, the appearance of carbon dioxide compound at 2363cm -1 can either be weak or medium. The frequency range for the absorption of O=C=O is 2400cm -1 -2000cm -1 . The wavenumber1090cm -1 and 803cm-1 show the stretching vibrations of Si-O-Si and Si-O respectively. The FTIR of the fourth sample heated at 8000C shows the absorption frequencies of 2369cm -1 , 1123-1027cm -1 , and 798cm -1 . The wavenumber 2369cm -1 shows weak or medium O=C=O stretching vibrations while 798cm -1 depicts the Si-O symmetric of quarts.  Figure 9 shows the FTIR result of the unheated fly ash. According to the analysis of this result, the wavenumber 2349 cm -1 shows the appearance of O=C=O stretching vibrations, 1049 cm -1 indicates the strong appearance of S=O stretching vibration of sulfoxide while 801 cm -1 depicts the Si-O symmetric stretching vibration of the quartz. The frequency range for the strong absorption of sulfoxide compounds is 1070-1030. It is observed that the unheated fly ash shows a strong appearance of O=C=O and S=0 stretching. All five samples (both heated and unheated) show O=C=O bending with very strong stretching vibrations for the unheated sample and those heated at 500 0 C and 600 0 C while the third and fourth samples heated at 700 0 C and 800 0 C show a weak or medium appearance of O=C=O stretching. This suggests that the more the temperature the greater the chances of eliminating the carbon content in the fly ash. Samples 1, 2, and 3 heated at 500 0 C, 600 0 C, and 700 0 C respectively also show the appearance of Si-O-Si but not observed for the unheated and the heated samples at 800 0 C. The Si-O quartz appears in all five samples according to the FTIR result. Finally, it is observed from the FTIR result of this work that sulfoxide functional group (S=O) only appears in the unheated fly ash and is not seen in all the heated samples. This is an indication that sulfoxide in the raw fly ash easily disappears when subjected to a certain amount of heat. The XRD result in figures 10 and 11 shows the presence of SiO 2 in all the fly ash samples. This information is in line with the SEM/EDS and FTIR characterizations. The results and analysis of all the characterization in this work confirmed that the purified fly ash obtained from Xi'an Linyuan Silica Limited is pure silica.

CONCLUSION
Critical analysis of the experimental results from the four characterization techniques shows that fly ash purely constitutes SiO 2 which is a significant constituent for the fabrication of refractory composites. According to the SEM/EDS results, the cenospheres discovered in the specimens are useful to refractory industrial engineers due to their low thermal conductivities and high heat resistance.