Application Technologies

#### **Chapter 5**

## Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed Gasifier

*Ali Can Sivri*

#### **Abstract**

Bubbling Fluidized Bed Gasifier (BFBG) technology is an efficient and economical way of producing syngas from various feedstocks, such as coal, biomass, and municipal waste. However, the prediction of the gasification process inside the BFBG is quite complex due to many factors, including multiphase flow hydrodynamics. This study analyzed the hydrodynamics of a bench-scale top-fed bubbling fluidized bed coal gasifier with sand or glass beads used as bed materials at different bed aspect ratios. Two separate test rigs were built with the same dimensions for cold flow (without reaction) and hot flow (with reaction) studies, respectively. The cold flow test rig was used to investigate the hydrodynamics of BFBG fluidization. Bed pressure drop, minimum fluidization velocity, and mixing were analyzed in the test room conditions. Following that, gasification tests were carried out in the hot flow BFBG test rig with a novel feeding system using the optimum hydrodynamical parameters determined from cold flow analyses. Results showed that syngas was successfully produced at an adequate composition. This study contributes to a better understanding of the fluidization hydrodynamics of the binary coal and bed material mixtures in a top-fed BFBG for a more optimum gasification process and easier operation of the BFBG.

**Keywords:** bubbling fluidized bed gasifier, coal fluidization, coal gasification, fluidization hydrodynamics, multiphase flow, synthetic gas

#### **1. Introduction**

The demand for energy has been growing due to the increase in human population and industrialization. However, the greater need for fuel and power generation brings more greenhouse gases and, hence, more environmental pollution, mostly because of the dominance of fossil energy sources being used as primary fuels for transportation and power generation. According to the International Energy Agency (IEA) and British Petroleum (BP), coal was the primary energy source, accounting for 38.5% of electricity production from 1971 to 2021 and 36% in 2021. Gasification can be an environmentally friendly alternative way of generating fuel (synthetic gas, syngas) from coal. Syngas produced by coal gasification can be used in various applications, such as transportation fuel [1], electricity production, heating, etc. Besides, the use of syngas can significantly lower greenhouse gas emissions. Bubbling fluidized bed gasifiers are a type of fluidized bed reactor that can significantly increase the gasification reaction efficiency compared to fixed-bed reactors. Also, it is more economical to operate and maintain compared to Circulating Fluidized Beds (CFB). However, the BFBG fluidization process is quite complex, and it has a direct impact on the reaction process and its efficiency. There are many parameters affecting the fluidization hydrodynamics inside the BFBG. Particle characteristics such as size, sphericity, density [2, 3], bed aspect ratio (the ratio of the bed height to the bed diameter) [3, 4], fluidizing gas and feedstock moisture content [5], temperature, and feeding location have strong effects on fluidization hydrodynamics. Besides, fluidizing gas velocity is a crucial parameter that mainly regulates and determines the fluidizing regime according to the particle characteristics and other factors. For an efficient gasification reaction inside a BFBG, all these parameters should be considered and adjusted interactively.

Particle size and density affect the bubble-induced particle mixing in a gas–solid fluidized bed. Because, particle shape affects the bubble formation and dynamics in fluidized beds. Spherical particles tend to form small and uniform bubbles, while nonspherical particles tend to form large and irregular bubbles [6]. Larger and denser particles tend to mix slower than smaller and lighter particles. Si and Guo [3] studied the fluidization behavior of binary mixtures of quartz sand with sawdust or wheat stalk in an acoustic bubbling fluidized bed. They found that the addition of sand improved the fluidization quality of the biomass particles. The authors suggested that the improved fluidization behavior was due to the increased particle density and reduced voidage caused by the addition of sand. They also observed that the fluidization behavior of the binary mixtures was affected by the particle size ratio and the proportion of sand in the mixture.

Higher aspect ratio beds have higher reaction efficiencies due to increased interparticle attraction and gas residence time, resulting in a longer reaction time. However, they lead to poorer mixing due to the transition from single bubble regime to slug flow regime [7]. Feedstock particles need to be delivered homogeneously along the bed height for optimum particle interaction and heat transfer rates. Thus, homogeneous mixing is suggested to obtain better syngas composition and higher reaction efficiencies. Most of the applications deliver the feedstock particles into the bed mainly by pushing the feedstock directly into the bed or on top of the bed by using a screw driver. However, driving a screwdriver directly into the reactor bed through the reactor wall can be expensive, and it is prone to mechanical and operational problems such as leaks. Zhijie Fu et al. [8] studied the particle mixing and segregation behavior in an Air Dense Medium Fluidized Bed (ADMFB) with binary mixtures of solid particles for dry coal beneficiation. The study examined the effects of various operating parameters such as particle density ratio, particle size ratio, mixture composition, superficial gas velocity, and fluidized bed height on the mixing and segregation pattern. The results of the study show that the degree of segregation increases with increasing density difference of binary mixtures and partial segregation can occur with an increase in particle size ratio. However, the mixing and segregation of binary systems are almost independent of lower excess gas velocity and initial bed height when it is over 15 cm. The study also employs a mixing index to evaluate the mixing and segregation performance and identifies criteria for good mixing to achieve the bed density adjustment.

During the operation of the BFBG, bed pressure drop and minimum fluidization velocity are the two main parameters analyzed to control the fluidization behavior

#### *Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

inside the fluidized bed. Particle density affects the minimum fluidization velocity and bed expansion of fluidized beds. Gao et al. [9] showed that higher particle density leads to higher minimum fluidization velocity and lower bed expansion. Abdullah et al. [10] investigated the effect of mixture bulk density and bed voidage on the minimum fluidization velocity (*Umf* ) of various materials, including Geldart B-group materials. They found that both mixture bulk density and bed voidage had a significant effect on the minimum fluidization velocity of the materials. In addition, they found that Geldart B-group materials exhibited better fluidization behavior compared to other groups of materials in Geldart's classification. On the other hand, many studies, conducted to analyze the effect of the bed aspect ratio on the *Umf* , show that higher bed aspect ratios do not affect the *Umf* significantly [11, 12]. Many predictions have been made to predict the *Umf* , but many of them do not agree, particularly for the binary mixtures (feedstock and inert materials). Besides, studies made in lab-scale applications can generate results that differ significantly in real (larger-sized) applications. In addition, most of these correlations have been generated in ambient conditions, excluding the temperature effect on fluidization hydrodynamics [13].

Temperature and pressure affect the hydrodynamics of gas–solid fluidized beds by changing the gas properties (density and viscosity) and the interparticle forces (van der Waals, electrostatic, etc.). Higher temperature and pressure increase the interparticle forces and cause agglomeration and defluidization of fine particles. The fluidization behavior of different Geldart groups of particles (A, B, and D) can vary significantly under extreme conditions [14]. Higher temperature leads to lower gas density and higher gas viscosity, which decrease the minimum fluidization velocity and increase the drag force [14, 15].

Overall, the mixing characteristics and quality of binary mixtures in fluidized beds can be an important aspect to consider in various industrial applications such as coal combustion, gasification, and catalytic reactions. According to the literature review, the theory behind fluidization hydrodynamics is not fully developed and understood. Hence, further research is needed to fully understand and optimize the mixing behavior of these complex mixtures under different operating conditions. Few studies considering the temperature effect on the fluidization characteristics and mixing behavior of binary mixtures of coal and inert materials in a fluidized bed have been found in the literature. As a result of the complexity of the fluidization and gasification theories, as well as the difficulties in their application, this technology is not widely used in both large- and small-scale applications.

This study aims to contribute to a better understanding of the coal and inert material (glass beads or sand) fluidization behavior in a top-fed deep-bed application of a BFBG by considering the particle characteristics, temperature, and bed aspect ratio.

#### **1.1 Gasification and bubbling fluidized bed gasifier**

Gasification is also known as "partial combustion" or "oxidation" due to the lower oxygen requirement (25 to 40%) compared to the amount of oxygen used in the stoichiometric combustion reaction. As a result, in addition to syngas, gasification generates some carbonaceous by-products such as ash, tar, and char (**Figure 1**). A series of reactions that take place interactively in a gasification reaction are shown in **Table 1**.

A bubbling fluidized bed gasifier is a type of Fluidized Bed Reactor (FBR) in which gasification takes place with a complex multiphase fluidization process of gas and

#### **Figure 1.**

*Gasification reaction diagram.*


#### **Table 1.**

*Major* 1st *step gasification reactions.*

solid particles. Fluidized bed reactors are more efficient in terms of heat transfer and carbon conversion rates compared to fixed-bed reactors, in which there is no particle motion or mixing [16, 17]. Fluidization can be described as the state of the solid particles that are suspended or gained motion (like fluidized) with the lift force exerted by an upcoming fluid (fluidizing fluid or agent) such as gas or liquid in a vertical column. If the flow rate of the gasifying agent is not enough for the desired fluidization operation, additional inert fluidizing gas can be used to obtain the required fluidizing gas flow rates. The mixing and the heat transfer rates are directly related to the bubble dynamics inside the BFBG. Here, besides the bubble dynamics, the inert (bed) material plays a significant role in providing the necessary heat transfer rates to the feedstock particles. Hence, inert material characteristics such as diameter, sphericity, density, and thermal conductivity affect fluidization hydrodynamics, and therefore, heat transfer rates and reaction rates are affected. The term "bed" is used to refer to the mixture inside the reactor. A sample fluidized bed reactor and 3D CAD model illustration are shown in **Figure 2a, b**, respectively. The main parts of the BFBG are: a plenum for fluidizing agent intake, a distributor plate to distribute the flow uniformly above it and provide the bed pressure drop for bubble formation, the reactor bed where the reaction takes place, and a freeboard to decrease the gas velocity to allow the particles to fall back to the reactor bed.

The operation of the BFBG can be described by the following procedures: Typically, a screw feeder mechanism is used to supply feedstock particles to the reactor bed from the bottom, side, or top. The gasification reaction starts when the carbonaceous particles are added to the oxygen-rich atmosphere at the required reaction temperatures. During the gasification reaction with the fluidization process, the

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

**Figure 2.** *Fluidized bed reactor diagram.*

produced ash particles sink to the bottom of the bed. On the contrary, light particles such as char can leave the reactor at high gas velocities. To increase the gasification reaction efficiency, particularly in circulating fluidized bed (CFB) applications, a cyclone is used to transfer the leaving particles back to the reactor bed. The operation of the BFBGs is more economical compared to CFB applications. Besides, BFBG can use a wider range of materials as feedstocks [18]. Before being used in the different applications previously described, the syngas is cooled and filtered.

#### **1.2 Bed pressure drop and minimum fluidization velocity**

The pressure drop between any points along the reactor bed height is equal to the weight of the particles between the measurement points per unit cross-sectional area of the fluidized bed. Thus, the bed pressure drop term is used for the pressure drop of the whole bed weight at the fluidization state. The bed pressure drop can be calculated by using the formula:

$$
\Delta P\_b A = W,\\
\text{where } W = mg = AH\_{mf} (1 - e\_{mf}) \left(\rho\_p - \rho\_\mathcal{g}\right) \mathbf{g} \tag{1}
$$

where, Δ*Pb* is the bed pressure drop, *A* is the cross-sectional area, *m* is the mass of the bed, *g* is the gravitational acceleration, *Hmf* is the bed height at minimum fluidization, *εmf* is the bed voidage at minimum fluidization, *ρ<sup>p</sup>* is the particle density, and *ρ<sup>g</sup>* is the fluidizing gas density. The bed voidage, *ε*, is the ratio of the void volume to the bulk volume of the bed, can be calculated as:

$$
\varepsilon\_1 = 1 - \frac{\rho\_b}{\rho\_s} \tag{2}
$$

where *ρ<sup>b</sup>* is the bulk density, and *ρ<sup>s</sup>* is the bed skeletal density.

The minimum fluidization velocity, *Umf* , is the gas velocity required to balance the bed weight and initiate fluidization. And, theoretically, as the gas velocity increases,

the pressure drop per bed weight remains constant. Many correlations have been developed to predict the minimum fluidization velocity; however, most of these correlations, particularly for binary mixtures, do not agree on the prediction results [19]. One of the well-known correlations to predict the minimum fluidization velocity derived by Ergun [20] is:

$$\frac{\rho\_{\text{g}}\left(\rho\_{\text{p}}-\rho\_{\text{g}}\right)\left(\text{g}d\_{\text{p}}^{3}\right)}{\mu\_{\text{g}}^{2}}=\frac{150\left(1-\varepsilon\_{\text{mf}}^{2}\right)}{\phi^{2}\varepsilon\_{\text{mf}}^{3}}\frac{\rho\_{\text{g}}U\_{\text{mf}}d\_{\text{p}}}{\mu\_{\text{g}}}+\frac{1.75}{\phi\varepsilon\_{\text{mf}}^{3}}\frac{\rho\_{\text{g}}^{2}U\_{\text{mf}}^{2}d\_{\text{p}}^{2}}{\mu\_{\text{g}}^{2}}\tag{3}$$

where *dp* is the mean particle diameter, *μ<sup>g</sup>* is fluidizing gas viscosity, and *ϕ* is the average particle sphericity.

Furthermore, the minimum fluidization velocity can be determined graphically by measuring the bed pressure drop as the fluidizing gas velocity increases. The intersection of the lines of the slopes in the fixed-bed state and the complete fluidization state, respectively, gives the minimum fluidization velocity. Fluidization starts at the initial fluidization velocity and reaches a complete fluidization state at the complete fluidization velocity. The graphical illustration of the determination of the initial (*Uif* ), minimum (*Umf* ), and complete (*Ucf* ) fluidization velocities is illustrated in **Figure 3**.

#### **1.3 Fluidization regimes**

The potential fluidization regimes within a fluidized bed with increased fluidizing gas velocity (*Ug*) are shown in **Figure 4**. The bed retains its shape and bulk density as long as the gas velocity remains below the minimum fluidization velocity (*Umf* ). This regime of fluidization is called "packed bed" or "fixed bed," which is illustrated in **Figure 4a**. With the rise in the gas velocity, initially smaller particles in diameter are suspended by the upcoming fluidizing gas. Later, with an adequate flow rate, all particles become lifted, and the weight of the bed is balanced with the force exerted by the fluidizing gas on the bed's cross-section area. The minimum fluidization velocity, *Umf* , is the gas velocity required to balance the bed weight and initiate fluidization.

**Figure 3.** *Graphical solution to determine Uif , Umf , and Ucf for increasing superficial gas velocity (fluidization).*

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

**Figure 4.** *Schematic illustration of the fluidization regimes.*

And, theoretically, as the gas velocity increases, the pressure drop per bed weight remains constant. The shift in the bed height (bed expansion) due to the minimum fluidization condition is demonstrated in **Figure 4b**. As *Ug* continues to rise, the bed gains momentum, and motion starts with the bubbles emerging, as seen in **Figure 4c**. Maintaining a homogeneous bubble distribution, bubble formation, and bubble frequency is critical for better mixing, which leads to higher heat transfer rates and, consequently, faster reaction rates. As the gas velocity increases, the bubbles condense into larger bubble formations known as "slugs" with a diameter similar to the bed diameter (**Figure 4d**). Bubble formations break up as *Ug* increases, resulting in rapid particle mixing (**Figure 4e**). A higher increase in the gas velocity causes a transition to a fast fluidization regime (**Figure 4g**). A further increase in the gas velocity can transport the particles outside of the bed, as seen in **Figure 4g**. This regime is called pneumatic transport.

#### **1.4 Geldart's particle classification**

The multiphase flow fluidization process is influenced by particle diameter, sphericity, and density. Geldart [21] classified four particle groups according to their fluidization behavior by measuring the variations in gas and solid phase densities with the mean particle diameter. Geldart group particles are classified as follows:

**Group A:** Particles in Group A, such as cracking catalysts, have a tiny diameter (20 to 100 μm) and/or low density. After the minimum fluidization condition, dense phase expansion is visible. As a result, Group A particles require higher gas velocities for bubble formation compared to Group B particles.

**Group B:** Group B particles have mean diameters and densities ranging from 40 to 500 μm and 1.4 to 4 g/cm3 , respectively. A good illustration of this category of

particles is sand. Bubbles are visible shortly after the minimum fluidization velocity. This group of particles shows the best fluidization characteristics.

**Group C:** Group C refers to particles having a diameter (10 to 80 μm) and a high degree of cohesion. Due to the increased interparticle forces caused by their high cohesive nature, they mix and fluidize poorly. Bubbles can be seen shortly after the lowest fluidization velocity, with a slight bed expansion.

**Group D:** Group D particles often have high particle densities and have diameters greater than 600 μm. Hence, they require higher gas flow rates to fluidize. Compared to Group B particles, they show poorer fluidization behavior.

#### **2. Experimental setup and methodology**

#### **2.1 Cold flow and BFBG test rigs**

The experimental setup consists of the cold flow (**Figure 5a**) and BFBG (**Figure 5b**) test stands. The cold flow test rig, made of transparent acrylic, allows the visualization of the fluidization behavior and the measurement of the pressure drop at ambient conditions. The main parts of the cold flow test rig are a plenum, a distributor plate, the bed section, and the freeboard (**Table 2**). A stainless steel distributor plate with a 10-μm pore size and 39% total porosity was used in the test rig. A mass flow controller was used to control and regulate the flow rate of the fluidizing gas (nitrogen, air). Pressure was measured at nine different points aligned vertically with

**Figure 5.** *a) Cold flow and b) BFBG experimental setups.*

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*


**Table 2.**

*Cold flow rig characteristics.*

3.81 cm increments along the bed height, starting just below the distributor plate (measurement point 1) up to the measurement point 9 just below the transition cone between the reactor bed and the freeboard. Pressure signals were recorded at a 10 Hz sampling rate using a data acquisition system. Later, the obtained data was analyzed with Python-based software. **Figure 5a** depicts the alignment of the pressure taps. Fluidization behavior was studied by using the images captured by a high-speed camera for each test case. Images were processed with an open-source image processing tool called Python-Scikit to improve their qualities and the contrast between the inert and feedstock materials to better visualize the mixing condition. Cold flow experiments were conducted for the total mixture masses of 100, 200, and 300 g with the feedstock (coal) on top (segregated state) with a weight ratio of 4% to simulate the actual BFBG hydrodynamical behavior at the elevated temperatures. For each test case, measurements were taken after waiting at least another 30 seconds to stabilize the test case and avoid transition data. Tests were repeated three times, and the results are shared in the Cold Flow Analysis section.

The BFBG test stand consists of the BFBG reactor, a top-load furnace, a screw feeder, and a micro gas chromatograph to analyze the gas composition of the synthetic gas acquired from the gasification tests. A BFBG reactor was installed inside the furnace, which can reach temperatures of 1500°C. The temperatures inside the reactor, just above the inert material, and on the wall were measured with K-type thermocouples. The maximum temperature measured during the experiments was around 820oC, which is adequate to provide the required heat transfer rates for a successful gasification reaction. The BFBG reactor was made of inconel steel with similar dimensions as the cold flow test rig. The BFBG test stand also measured pressure drop to investigate the effect of elevated temperatures on bed pressure drop. Tests were conducted at the same time interval for the 200 and 300-g unary sand mixtures. The study conducted by Sivri [3] contains a detailed description and information about the design of the test stands.

#### **2.2 Material analysis and preparation**

In this study, the cold flow and actual BFBG tests used coal as the feedstock and sand or glass beads as the bed material, respectively. It was Pittsburgh coal seam number eight that was used as feedstock. Glass beads from Ballotini and commercialgrade fine silica sand from Quikrete brands were used as bed materials. The results of moisture, volatile, ash, and elemental analyses of the biomass and coal are displayed in **Table 3**. Further, the size and sphericity analyses with their distributions were


#### **Table 3.**

*Elemental and proximate analysis (by mass) of bituminous coal.*


**Table 4.**

*Material size, density, sphericity, and Geldart's group analysis.*

conducted with a dynamic image analysis method (Sympatec GbmH, Model QICPIC) and skeletal densities were analyzed by a gas pycnometer (AccuPyc, Model 1330 Helium Pycnometer). The results of the size, sphericity, and density analyses for the feedstock and inert materials are summarized in **Table 4**. Glass beads showed the highest mean sphericity of 0.93 compared to 0.85 for coal, and 0.86 for sand, respectively. Sand and glass beads have a narrower sphericity (90% between 0.85 and 0.95) and size (90% between 235 and 347 μm) distribution compared to coal as well. Coal has a bigger mean diameter of 362 μm compared to glass beads (271 μm) and sand (324 μm). Hence, the coal and glass beads mixture has better packing, which enhances the fluidization characteristics (**Figure 6**).

**Figure 6.** *Particle size and sphericity distributions.*

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

#### **3. Cold flow analyses**

BFBG deep-bed applications have a higher gasification efficiency due to improved heat transfer rates and a longer gas residence time. However, top-fed deep-bed application of the BFBG is quite complicated, not only because of the requirement to deliver the feedstock particles homogeneously on top of the bed, but also because of the requirement for an optimum homogeneous binary mixture to obtain the most efficient gasification process. Hence, the observation of the fluidization process in cold flow conditions is required to better understand the intricate hydrodynamics of coal-top-fed deep-bed *Hp=Db*≳2 binary mixtures.

#### **3.1 Mixing and fluidization behavior of coal with sand or glass beads mixtures**

The mixing and fluidization behavior of binary mixes of coal with two separate inert components (glass beads and silica sand) was analyzed in this section. The total masses of the binary mixtures investigated were 100, 200, and 300 g, respectively, including coal, which made up 4% of the total mass and was spread on top of the bed material. Bed pressure drop, Δ*Pb*, and minimum fludization velocity, *Umf* , were analyzed as a function of superficial fluidizing gas velocity with a 0.0146 m/s (1 SLM) increments. The bed fluidization and mixing behavior were also observed with the high-speed camera at each flow rate after attaining fluidization. In **Figures 7**–**12**, each column picture under the pressure drop curve represents the bed behavior at the corresponding fluidizing gas velocity. With the use of these images, the ideal fluidizing-gas velocity interval was estimated to obtain an almost homogeneous (quasi-homogeneous) binary mixture with reliable fluidization behavior for the BFBG operation.

Coal and glass beads binary mixtures have higher bulk density (1*:*61 g*=*cm<sup>3</sup> >1*:*56 g*=*cm3) and higher average sphericity (0.93 > 0.86) (**Table 4**) compared to coal with sand mixtures. Hence, better fluidization and mixing behaviors were expected for the coal and glass beads mixtures. Later on, Case I and Case II represent the coal and glass beads mixture and the coal and sand mixture, respectively. The results obtained during the investigation of the mixing and fluidization behavior of the binary mixtures are shared in this section.

The bed pressure drop and fluidization behaviors of the binary mixtures of Case I and Case II for the total mixture mass of 100 g with increasing fluidizing gas velocity are shown in **Figures 7** and **8**, respectively. The initial static bed aspect ratio (*Hp=Db*) measured as ≈1*:*83 for Case I, and slightly higher ≈2 for Case II due to the lower average sphericity and bulk density. In Case I, there was a smooth transition from the minimum fluidization to complete fluidization after the pressure curve climbed linearly up to the initiation of the fluidization. However, the transition was not smooth in Case II. The abrupt drop in bed pressure drop was followed by an increase in *Ug* , and Δ*Pb* rose until all particles fluidized as a result of Case II's greater particle size distribution and channeling. Tiny bubble formations due to channeling before complete fluidization were visible in Case II at the speeds between 0.117 and 0.146 m/s (**Figure 8b** columns 5–7). Besides, wider particle size distribution and less sphericity encoupled with the humidity effects of Case II caused to reach minimum fluidization velocity at a higher speed of 0.06 m/s compared to Case I with a *Umf*≈0*:*05 m/s. As seen in **Figure 7b**, except for a tiny layer of coal on top of the glass beads in Case I, the bed was almost in a well-mixed state for the fluidization gas velocities of 0.088 and

#### **Figure 7.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 100 g.*

0.103 m/s (**Figure 7b** third and fourth columns, respectively). A further increase in the flow rate resulted in smooth bubble formations and eventually a well-mixed state at around *Ug* ¼ 0*:*12 m/s. However, Case II required a higher *Ug* of 0.16 m/s to achieve a well-mixed state (**Figure 8b** eighth column) due to the same reasons mentioned in the *Umf* comparison for both cases. In Case II, mixing happened just after reaching the complete fluidization velocity (*Ucf* ) of ≈0*:*14 m/s because of the sudden breakdown of the interparticle and cohesive forces. At higher gas velocities after reaching the complete fluidization, flat slug formations were not observed due to the low bed aspect ratios, which were around two for both cases.

Cases I and II were also tested for the total mixture mass of 200 g with the coal making up 4% of the total mass. **Figures 9** and **10** demonstrate the fluidization

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

#### **Figure 8.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 100 g.*

behavior and pressure drop changes with increasing gas velocity for Cases I and II, respectively. Static bed aspect ratios *Hp=Db* were 3.5 and 3.66 for Cases I and II, respectively. In both Cases, the bed pressure drop showed a linear increase during the fixed-bed state. As in the previous test, the transition from fixed bed state to fluidization was smoother for Case I compared to Case II due to the reasons mentioned earlier. Furthermore, in Case I, mixing began earlier at *Ug*≈0*:*074 m/s, just after reaching the complete fluidization, with the penetration of glass beads into the coal layer. *Ug*≈0*:*088 m/s achieves a nearly well-mixed state, except for the tiny coal layer on top of the bed. But, in Case II, complete fluidization was achieved relatively late at a gas velocity of *Ug*≈0*:*09 m/s, and complete mixing could be achieved at the gas velocity of 0.103 m/s with a narrow coal layer on top. Another factor, except the sand characteristics, that contributed to the delay in reaching the complete fluidization and

#### **Figure 9.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 200 g.*

well-mixed state was the higher relative humidity of the fluidizing gas (air), which strengthened the interparticle forces and cohesiveness. In both cases, *Umf* was measured graphically around 0.05 m/s. Further increase in *Ug* caused slug formations, which can be seen in both Cases. Despite the slug formations, the well-mixed states achieved in both Cases (**Figure 9b** and **10b**, columns 4–11), supported by the same images, demonstrate almost a homogeneous distribution of the coal particles inside the bed material. Some of the coal dust particles were stuck on the bed wall in Case II due to the higher ambient humidity.

Later on, experiments were conducted to test the fluidization behaviors for a deeper bed with bed aspect ratios of 5.16 and 5.66 and a total mass of 300 g, with the coal making up 4% of the total mass for Cases I and II, respectively. Fluidization behaviors and pressure drop attitude with increasing gas velocity for Cases I and II

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

#### **Figure 10.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 200 g.*

with the total mass of 300 g are shown in **Figures 11** and **12**, respectively. Similarly, bed pressure drop increased linearly during the fixed-bed state for both Cases as in the previous test conducted with the total masses of 100 and 200 g, respectively. Despite the higher bed aspect ratio compared to the previous tests, Case I still showed a smooth transition from static state to fluidization as seen in **Figure 11**. On the contrary, transition was relatively stiff for Case II. Due to the stronger interparticle forces and cohesiveness in Case II peak pressure drop value obtained was higher (≈3*:*3 kPa) compared to Case I (≈2*:*4 kPa). After reaching the peak pressure drop in Case II, there was an abrupt fall to ≈2*:*4 kPa. Case I and II reached the complete fluidization states at the approximate gas velocities of ≈0*:*07 m/s and ≈0*:*075, respectively. For both Cases after reaching the complete fluidization, coal particles started mixing with the sand particles at the contact of the segregated layers. Complete mixing is achieved at an approximate gas velocity of 0.09 m/s for Case I, and 0.1 m/s for Case II. As in the

#### **Figure 11.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 300 g.*

previous experiments conducted for 100 and 200 g total masses, a well-mixed state was achieved at a higher gas velocity in Case II compared to Case I for 300 g due to the same reasons mentioned before. Slug formations were visible for both Cases after the gas velocity of ≈0*:*1 m/s (**Figure 11b** and **12b**, columns 4–11). For both Cases, the bed could preserve its well-mixed state at higher gas velocities despite the narrow coal layer on top of the bed. Elutriation was observed at a gas velocity of 0.175 m/s and higher for Case II, and 0.207 m/s for Case I.

#### **3.2 Summary and conclusions**

The fluidization and mixing behaviors were investigated for Cases I (coal and glass beads) and II (coal and sand) mixtures with different total masses of 100, 200, and

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

#### **Figure 12.**

*Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 300 g.*

300 g, with coal making up 4% of the total mass. Initially, the mixtures were in a segregated state in which coal was placed on top. The fluidization and mixing behaviors of the mixtures were observed as the gas velocity increased. The main conclusions drawn from this study were:


#### **4. Fluidization and gasification analyses at elevated temperatures**

The effect of temperature on fluidization characteristics such as bed pressure drop and minimum fluidization velocity was investigated, and the results are shared in this section for sand material. Besides, gasification test results for the binary mixture of coal and sand are shared.

#### **4.1 Effect of temperature on the bed pressure drop and minimum fluidization velocity**

The bed pressure drop versus gas velocity was studied for unary sand mixtures of 200 and 300 g at elevated temperatures ranging from 200oC to 805oC (**Figure 13**). The bed aspect ratios measured were 3.3 and 4.83 for the total masses of 200 and 300 g, respectively, in the cold flow test rig, with similar dimensions to the BFBG.

**Figure 13.** *Bed pressure drop versus fluidization gas velocity at different temperatures. G.*

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

Bed pressure drop increased linearly at all temperatures for both total bed masses of 200 and 300 g, respectively. A linear increase in the pressure drop was followed by a smooth transition to complete fluidization in all cases. However, for the mass of 300 g, small pressure drop peak formations were visible due to higher wall effects. After complete fluidization, the pressure drop continued to increase with the increasing gas velocity due to three main factors: wall effects, increased bed voidage [22], and stronger interparticle forces particularly for this narrow-sized (3.81 cm diameter) deep-bed reactor. Similar results of increasing bed pressure drop due to wall effects were also reported by Srivastava and Sunderasan [23] and Olatunde et al. [24]. While the minimum fluidization condition's bed pressure drop increased with temperature, *Umf* decreased for both total masses of 200 and 300 g (from 0.086 to 0.063 m/s for 200 g, and from 0.096 to 0.062 m/s for 300 g). The decrease in the *Umf* can be attributed to the higher interparticle forces [22], and lower *Re* numbers at higher gas temperatures, which lead to higher gas viscosity and increase the drag coefficient.

#### **4.2 Gasification analysis**

The findings of the gasification of coal with 10% steam and air in the BFBG are presented in this subsection. The elemental composition of coal (**Table 3**) reveals that it has a low oxygen content, demanding the use of an outside oxygen source to create the appropriate syngas composition. As a fluidizing agent, a mixture of air and steam was used. And sand was preferred due to its lower thermal conductivity for a better thermal management of the reactor. When 10% steam was added to the coal feed, an average ratio of H2*=*CO ¼ 3*:*23 was observed ignoring the nitrogen content. Gasification tests were performed in a slightly fast fluidization regime to assure sudden mixing and enhanced particle-particle and particle-gas interaction, which generates better heat transfer between the bed material (sand) and the feedstock (coal) particles and improves reaction kinetics. The complex gasification reaction kinetics interact strongly with fluidization hydrodynamics, particularly bubble formations; shape, frequency, and so on. As a result, fluidization hydrodynamics should be studied in greater depth in order to better understand the complicated process of gasification and its interaction with fluidization hydrodynamics. Initially, the temperature of the bed medium was elevated to the desired temperatures (around 800oC) to achieve an effective gasification process. Temperature was measured on the reactor wall, at the level of the bed material, and inside the bed, just above the bed material, to assure an accurate temperature measurement to start the gasification process. Later, for each supply, 2 g of coal were fed by the supply line using pressurized nitrogen. Coal flow rate was maintained at the rate of 2 g/min. The average syngas composition at the initial stage of the pyrolysis, that liberates hydrogen component in coal first before proceeding the carbon gasifying status, and its low heating value obtained during the tests are shared in **Table 5**.


**Table 5.** *Syngas composition and low heating value.*

#### **4.3 Summary and conclusions**

The effect of the temperature on the BFBG fluidization hydrodynamics was analyzed and the initial gasification test results were shared in Chapter 4. The main conclusions are:


#### **Acknowledgements**

This work was supported by US Department of Energy's Fossil Energy Advanced Gasification Program. The Research was executed through NETL Research and Innovation Center's Advanced Gasification effort within the Advanced Reaction Systems FWP under the RSS contract 89243318CFE000003. The author gratefully acknowledge WVU's CIGRU, MAE, and CBE departments for their assistance with comprehensive mechanical, electronic, and data-acquisition hardware/software systems, respectively. The research was conducted at the WVU's Advanced Combustion Laboratory in Morgantown, WV.

#### **Disclaimer**

This work was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with Leidos Research Support Team (LRST). Neither the United States Government nor any agency thereof, nor any of their employees, nor LRST, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

*Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed… DOI: http://dx.doi.org/10.5772/intechopen.112118*

### **Author details**

Ali Can Sivri Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, USA

\*Address all correspondence to: acs0031@mix.wvu.edu

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications

*Henilkumar M. Lankapati, Kalpana C. Maheria and Ajay K. Dalai*

#### **Abstract**

The generation of a large quantity of waste coal fly ash (CFA) *via* coal combustion process during power generation is of major concern as disposal of such huge quantity of fly ash causes serious threats to the environment. There is an exigent need to find out the proper solution for its disposal/utilization to reduce its harmful effects. The composition of waste coal fly ash mostly consists of silica and alumina. Hence, the researchers are tempted to utilize waste coal fly ash as a starting ingredient to make value-added materials like zeolites. It is anticipated that such research efforts will act as a valuable aid to reduce the disposal cost of fly ash and ultimately reduce harmful effects of fly ash to the environment. In this review, various synthesis methods to synthesize different types of zeolites from CFA, such as Zeolite-A, Zeolite-X and Zeolite-P, have been summarized and their potential for various applications such as sorption and catalysis has been explored.

**Keywords:** fly ash, waste utilization, zeolites, catalysis, sorption

#### **1. Introduction**

Environmental issues are the most burning issues in the world nowadays. The leading industrialist, scientists, researchers, and environmentalist around the world strive hard for the finding of the proper solution to mitigate or reduce the impact of various environmental problems. The rising issues and researches concerning around the world are (i) to reduce CO2 levels by utilizing renewable sources as fuels and for the energy production, (ii) storage and disposal of the waste coal fly ash generated from the thermal power plants, (iii) to identify the various contaminants such as toxic/heavy metals, pesticides, petroleum hydrocarbons, which deteriorate the air, soil, and water environments, (iv) to reduce the generation of various pollutants by adopting green chemistry principles, and (v) to find out the best treatment technologies for the removal of contaminants and recovery of the precious metals or products from the waste materials or to convert the waste materials to value-added materials.

Nowadays, environmental researchers and scientists focus on the utilization of waste for the treatment of waste, that is, converting waste materials into value-added materials such as adsorbents and catalysts and utilize them for the effective treatment of the waste. The waste coal fly ash (CFA) is generated as a byproduct during the electricity generation in thermal power plants. The CFA is considered as the harmful materials, and it is generated in a vast quantity that their disposal is so crucial and required large acre of land. The composition of waste CFA mostly consists of silica and alumina along with trace amount of other rare earth elements, which are precious. Hence, the researchers are tempted to utilize waste coal fly ash as a source of precious elements and make value-added materials such as zeolites from it. Synthesis of different types of zeolites from CFA, such as zeolite X, P, and A, is reported in the literature [1, 2].

Few studies have been undertaken for the applications of CFA-derived materials as the environmental remediation, such as, removal of phosphates and nutrients from the water [3]. Deng et al. have reported the method of converting CFA into materials like adsorbent using microwave-assisted alkali modification and utilized to adsorb Cr6+ metal ions from the water [4]. Appiah-Hagan et al. have modified waste CFA using freezing and thawing method and studied the adsorption efficiency of modified fly ash for the sorption of Pb2+, Co2+, Ni2+, Cu2+ Cr3+, and Cd2+ [5]. Similarly, lime-activated fly ash is utilized for the sequestration of heavy metal ions, such as Zn2+, Pb2+, and As5+ from the aqueous solution [6]. Chinh et al. have explored the potential of fly ash modified with (3-mercaptopropyl) triethoxysilane for the scavenging of Hg2+ ion [7]. Lathiya et al. have converted waste CFA into sulfated fly ash and applied it as a solid acid catalyst for the biodiesel production from maize acid oil [8]. These studies reveal that fly ash can be utilized efficiently for its conversion into valuable materials and may be used further for metal extraction from aqueous solution.

#### **2. Fly ash generation and utilization**

#### **2.1 Fly ash generation**

Thermal power plants are the main sources of the waste CFA generation as a byproduct of electricity generation. According to the bp (British Petroleum) statistical review of world energy 2022 (71st edition) published by British Petroleum, the generation of electricity is increased by 6.2% during the year 2021. Along the total generation of electricity, coal remains the dominant as a fuel with 36% of its share followed by natural gas (23%), hydroelectricity (15%), renewable sources (13%), nuclear energy (10%), and others (1%) as shown in **Figure 1** [9].

The last 6-year scenario of the electricity generation by fuels as shown in **Figure 2**, reveals that the coal remains dominant (approx. 36%) over all other sources since last few years [9]. The coal utilization ultimately leads to the generation of huge quantity of waste CFA, and its disposal imposes threat to the environment due to its hazardous nature. Though energy generation modes are shifting toward renewable sources, still modes of thermal energy production dominate over the renewable sources as these technologies are under development stages. Hence, the dependency on the coal for energy production is still expected to dominate significantly in the upcoming years also. This will lead to generation of significant amount of fly ash, and there is an urgent need to develop the effective utilization modes of fly ash, which are economically and environmentally viable.

In India, around 76% of total energy demand is supplied by the coal-based thermal power plants. There are more than 202 coal-based thermal power plants in India, with *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

#### **Figure 1.**

*World electricity generation by fuels during the year 2021 [9].*

#### **Figure 2.**

*Scenario of world electricity generation by fuels (terawatt-hours) from 2016 to 2021 [9].*

installed capacity around 209,990 MW [10]. According to the CEA (Central Electricity Authority) report, August 2021, these power plants consume 686 million tonnes of coal and produce 233 million tonnes of waste CFA during the year 2020–2021. Annual fly ash generation from Indian coal power plants in last 10 years rose from 145.4 million tonnes during the year 2011–2012 to 232.6 million tonnes during the year 2020–2021, an increase of almost 60% [10].

According to the initiative implemented by Ministry of Environment, Forest and Climate Change (MoEFCC), Government of India, to utilize the 100% waste CFA, around 215 million tonnes of fly ash is utilized during the year 2020–2021, which is around 92.4% of total fly ash generation and 7.6% (108.6 million tonnes) of fly ash

**Figure 3.** *Trends of CFA generation and utilization in India from 1996 to 2021 [10].*

remains unutilized [10]. **Figure 3** shows the trends in fly ash generation and utilization in India from the year 1996 to 2021.

#### **2.2 Legacy ash**

The build-up of huge quantity of waste CFA by thermal power plants over decades or other terms, the fly ash stored in the ash ponds, or ash dykes by thermal power plants are considered as the legacy ash. From the CEA report, the available total quantity of legacy ash as on March 31, 2021 is 1738.19 million tonnes, which is dumped or stored at various ash dykes occupying large acre of land area surrounding the power plants [10].

According to the present practice in India, the requirement of land area for the disposal of coal fly ash by the year 2022 is estimated about 126,000 ha (approximately 0.6 ha per MW), if effective utilization will not be implemented. Even 92.4% fly ash is utilized, the land required for unutilized fly ash during the year 2020–2021 (108.6 million tonne) would be around 58,800 ha as per the latest reported data by CEA [10, 11].

#### **2.3 Policy and notifications**

MoEFCC, Government of India, have taken various initiatives for the protection of land to be polluted by the fly ash slurry, which is disposed into the ash dykes or ash ponds. In their effort to reduce the requirement of large acre of land, MoEFCC has issued first notification regarding the fly ash utilization on September 14, 1999, which was further amended time to time over the years, 2003, 2009, 2016, and 2021. The first notification includes the enforcement of CFA utilization for the manufacturing of bricks, tiles, blocks, and other construction-related materials, and manufacturer within 50 km radius of coal-based thermal power plant must mix at least 25% of CFA with soil by weight percentage [12].

In the first amendment, the radius of the availability of bricks/tiles manufacturers, who have to follow the notification was amended and expanded up to 100 km from

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

50 km. It further describes target for the waste fly ash percentages, which needs to be utilized by manufacturer as per the radius wise, up to year 2007 [13].

The amendment notification issued on November 3, 2009 provides the details of minimum quantity of fly ash, which needs to be utilized for building materials according to the category of fly ash based product. It also provides targets to the thermal power plants (those are already in operation before the date of this notification) for the utilization of fly ash in a phase manner to reach 100% utilization of fly ash, that is, starting from the date of the notification, target of 50, 60, 75, 90, and 100% for the first, second, third, fourth, and fifth, respectively, while for the newly constructed power plant, the target of achieving 50, 70, 90, and 100% fly ash utilization for the first, second, third, and fourth years, respectively, from the date of the commissioning of power plant [14].

Further, in the year 2016, the new amended notification issued in order to widen the scope of utilization and the vicinity of radius from 100 km to 300 km of the availability of manufacturers or builders needs to use the fly ash in building materials. It also stated that the cost of transportation of CFA to the manufacturers or builders within the 300 km radius shall be borne by the respective power plants and beyond 300 km, the cost shall be equally shared between the users and power plants. This notification also prescribes power stations to upload the details of stock available with them and keep updated regularly [15].

The latest notification issued on April 22, 2021, is focused on the utilization of legacy ash. According to the notification, thermal power plants need to utilize the legacy ash within 10 years from April 1, 2022 as stated in the notification [16]. The percentage utilization of legacy ash shall be based on the annual fly ah production, that is, the utilization of legacy ash should be at least 20% within 1st year; 35% within 2nd and 50% above 3rd to 10th year, from the issuance of the notification, failing to this will impose penalty as per quantity of unutilized legacy ash [16].

#### **2.4 Fly ash utilization**

Research scientists all around the world strive hard for the development of various modes of fly ash utilization in cost-effective and ecofriendly manner. Due to their immense efforts, the fly ash utilization increased steadily from the 6.6 million tonnes (1996–1997) up to the 215 million tonnes (2020–2021) [10]. **Figure 4** shows the details of the fly ash utilization in India during the year 2020–2021.

**Figure 4** reveals that the considerable amount of fly ash is utilized by the cement sector, which is 25.8% of the total fly generated during the year. Then, other significant amount 15.6% of fly ash was utilized for the reclamation of low-lying area, followed by the sector of roads and flyovers, where around 15.0% of fly ash utilized. Bricks and tiles manufacturer have used about 12.9% of fly ash in 2020–2021, while ash dykes raising and filling of mining area utilized about 6.2 and 0.83%, respectively, of fly ash generated. Only 0.03% fly ash was utilized in the sector of agriculture and hydro-power sector. The quantity of fly ash remains unutilized during the year 2020– 2021 was 7.59%, which is around 108 million tonnes [10].

Restogi et al. have studied the potential to utilize fly ash in the field of different construction area and came out with the results for the fly ash utilization with the projected level until 2030. The study revealed that concrete and cement sectors have reached their threshold limit with accommodating around 35 to 40% of total fly ash generation in the forthcoming years, while the sectors of bricks and blocks are found to be the most potential mode for the fly ash utilization, with the capacity to

#### **Figure 4.**

*Fly ash utilization in India during the year 2020–2021 as per CEA report, Aug-2021, Ministry of Power, INDIA [10].*

accommodate all unutilized fly ash remaining during the year along with the potential to utilize the legacy ash within the next 15 to 20 years [17].

Though sectors such as mine filing (6.2%), reclamation of low-lying area (15.6%), ash dykes raising (7.9%), and roads and flyover (15%) consume ample amount of fly ash, it should be consider as the last option as these are still existence of threats to the environment after utilization of these modes. Hence, there is still huge quantity of fly ash available to be explored for its efficient utilization in such a manner so as to remove or reduce their harmful effects to the environment. These modes of utilization are followed by the manufacturer as the regulations have been enforced to utilize fly ash in these sectors as compulsion. If the mode of utilization that utilizes the fly ash as raw materials to produce value-added materials, then it will be utilized efficiently by the various industrialists to produce the value-added products, such as the precursor for the ceramics industries and development of ceramic membrane, for the development of adsorbents to remove toxic metals, drug impurities from the water, and development of catalysts such as zeolites for various organic transformations like methane to olefin conversion, biodiesel production, fast pyrolysis of Jatropha waste [18–25]. As fly ash consists of various elemental oxides, specifically SiO2 and Al2O3, which are useful as precursors for the zeolites. Hence, intense research works have been carried out for the development of zeolites from the waste CFA [18, 26].

#### **2.5 Properties of fly ash**

The fly ash can be classified as two different types of classes (Class F and Class C) according to the ASTM C-618-3 standards, which are shown in the table below [27]:

• Class F: The fly ash produced after burning of the anthracite and bituminous type of coals. This kind of fly ash contains minimum 70% of silicon dioxide (SiO2) plus

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

aluminum oxide (Al2O3) plus iron oxide (Fe2O3) along with CaO less than 10% and possesses pozzolanic properties.

• Class C: The fly ash that produced after burning of lignite or sub-bituminous type of coals. This type of fly ash contains minimum 50% of silicon dioxide (SiO2) plus aluminum oxide (Al2O3) plus iron oxide (Fe2O3) along with lime (CaO) content more than 10% (mostly found in the range of 15 to 20%, sometimes up to 40%) and possess cementitious properties along with pozzolanic properties [27–32].

The Indian coal fly ashes with low lime content are relatively higher in concentration of oxides of silica and alumina, whereas oxides of iron contents are found lower. Due to such properties, these fly ashes require higher temperature for fusion because at lower temperature the chances of glass formation are also low. In these types of fly ashes, the silica content is found almost double than the content of alumina, whereas in high-calcium fly ashes, the oxides of silica and alumina are found almost close to each other along with the significant amount of oxides of iron with respect to the lowlime fly ashes [30]. The heterogeneity studies of the fly ashes by various methods such as sieving, sink-float, and magnetic separation suggests that the heterogeneity in fly ashes with high lime contents is found higher as more variations in the compositions are observed in such kind of fly ashes [30, 33]. The general composition of CFA from various sources is summarized in **Table 1**.

Based on physical properties of the fly ash, it could be considered as fine glass powder with the regular spherical particles sizes in range of 0.5 μm to 100 μm. Such spherical particles shapes allow them to flow and blend freely in the mixture of concrete or cement. The fly ash particles possess ball bearing effect, which provide lubricating actions in the concrete with the plastic state and thus help to decrease the dry shrinkages [11, 34]. The concrete blended with fly ash tends to resist attack of water, sea water, mild acids, and sulfates, which ultimately increases the durability of the concrete mixture and makes it suitable for coastal environment [35, 36]. The fly ash having long-lasting pozzolanic properties, which is useful to tie up free lime, thus reduces the bleed channels by decreasing the permeability of the concrete. Such properties are also useful for increasing the structural strength of the concrete mixer over a time. Due to such kind of physical properties, around 27% of the fly ashes are utilized in the concrete and cement sector [34]. But, it is believed to be at the saturation level to be more utilized in this sector and needs to be explored the utilization of fly ash in other sectors, which turns it into value-added materials.

#### **3. Fly ash utilization for the synthesis of zeolites**

#### **3.1 Reported synthesis protocols for fly ash-derived zeolites**

#### *3.1.1 Hydrothermal synthesis method*

The zeolite formation most commonly proceeds through the hydrothermal synthesis process. The crystallizations or formation of zeolites proceeds through the hydrothermal synthesis process. The various parameters, such as temperature of crystallization, pH of gel, concentration of alkaline cations, time of crystallization, reaction conditions such as static or continuous stirring mode and autogenous pressure involved during the hydrothermal process, may define the formation of specific


#### **Table 1.**

*Chemical composition of coal fly ash (CFA) obtained from various sources.*

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

types of zeolites with their unique properties. The gel composition used to crystallize zeolites mainly contains the sources of hetero elements (tetravalent or trivalent elements), mainly silica and alumina, which, further, takes part in the formation of framework structure of zeolites. The gel also comprises of the sources of mineralizing agents such as OH and F along with inorganic cations, organic species, and solvent (generally water), which solubilizes the reactive elements in the gel and enables them to transfer into the growth of the zeolite crystals [37].

In hydrothermal synthesis of zeolite, homogenized well-mixed sol-gel is prepared by mixing precursors of silica and alumina, source of inorganic cations, and mineralizing agent (i.e., NaOH or KOH) along with the water as a solvent. The gel is then transferred to a teflon-coated autoclave reactor (static or agitated) and then heated for specific period of time with or without stirring [18, 37, 38]. As the hydrothermal crystallization proceeds the high temperature and autogenously generated highpressure environment triggered, the process of crystallization and the crystal growth proceeds according to the available environment to produce zeolite product. The parameters affecting the crystallization are the concentration of alkali, temperature of crystallization, time of aging, and liquid-to-solid ratio in the reactor. If the alkalinity is too low in the gel, there may be not enough Na+ availability too trigger the crystallinity, while, if the alkalinity is high, the dissolution of the crystal nucleus would be accelerated, which further affects the crystallization of the zeolitic product [39, 40]. The aging plays an important role in the crystallization process. Aging promotes the formation of oligosilicate ions by the polymerization of the single silicate ions available in to the sol gel, which is further utilized for the crystallization of the zeolite. The temperature of the hydrothermal synthesis is another important parameter for the efficient crystal growth of zeolite. The autogeneous pressure is dependent on the temperature and volume of the gel mixture of the autoclave reactor. High temperature of the reactor triggers the autogeneous pressure in the reaction vessels and thus accelerates the speed of the crystallization of zeolites. Different kinds of zeolite crystals are observed at different temperatures. If the temperature is too low during the process, the pressure will be less, thereby leading to reduce crystallization speed and difficulty in the formation of targeted zeolite crystals [41, 42]. Chang and Shih have synthesized zeolite A and faujasite from Class F type fly ash at low temperature (38 and 60°C, respectively), but required longer reaction time (more than 3 days) for the crystallization of Zeolite A, while at 90°C Zeolite P formation is observed [43]. On the other hand, Yoshida and Inoue have synthesized the similar zeolite at 90°C but it is observed that at the higher reaction temperature, phase of Zeolite A started to disappear and formation of Zeolite P is observed [44]. Amoni et al. have synthesized zeolite A from CFA *via* hydrothermal process after applying magnetic treatment to remove magnetic fraction and acid treatment to remove other unwanted elements [45]. Kumar and Jena have synthesized highly crystalline zeolites with pure phase of zeolites Na-P1, hydroxy sodalite, and analcime zeolites by direct hydrothermal synthesis method, with and without homogenization of the gel precursors, at 150°C temperature and 60 h (to obtain Na-P1) and 20 h (to obtain hydroxy sodalite with homogenization and Analcime without homogenization) [46].

The liquid-to-solid ratio (LSR) should be maintained in accordance with the reactor capacity to achieve desired product. The optimum LSR is useful to generate desired autogeneous pressure, which ultimately triggered the crystallization process. It is also useful to maintain Na<sup>+</sup> concentration and the alkalinity of the gel composition. Generally, the optimum LSR is 66% to get enough autogeneous pressure during the hydrothermal process [40, 47]. The optimum aging time helps to produce stable

silicon aluminum gel polymer, which further helps to get well-crystallized zeolite and also helps to reduce the time of crystallization. On the contrary, longer aging period may dissolve the colloidal skeleton, which affects the quality of crystallinity of the zeolites [48]. The hydrothermal synthesis methods suffer with few disadvantages such as it consumes high energy, longer reactions time, and sometimes suffering from lower crystallization efficiency [38]. Therefore, there is a need to explore other green synthesis approaches and pretreatment methods to prepare sol-gel materials before hydrothermal synthesis. The hydrothermal synthesis process is mostly followed for the zeolite crystallization, but to get targeted materials few pretreatments of the raw materials are necessary to improve the crystallization of pure zeolites.

Before the hydrothermal synthesis process, the pretreatment of the raw materials (specifically when waste CFA or other kind of waste raw materials is used as silica and alumina sources) favors the formation of the desired zeolitic materials. Such pretreatment methods involve the steps such as acid treatment of the waste CFA to remove undesired elements, that is, Fe2O3, CaO, MgO, K2O, and Na2O [49–51].

#### *3.1.2 Alkali fusion method*

The alkali fusion of the raw materials is another pretreatment method involving the fusion of the waste materials to extract maximum silica from it, which is further utilized to prepare gel composition to synthesize the zeolites *via* hydrothermal synthesis method. Fly ash is utilized as the sources of silica and alumina, which are the important starting ingredients for the synthesis of alumino-silicate or zeolite. Shigemoto et al. have introduced the pre-alkali fusion treatment prior to the hydrothermal synthesis process, to convert entire CFA particles into the zeolite [52]. The silica and alumina are present in the fly ash as the quartz and mullite phases. These phases of silica and alumina are less reactive as they are not easily soluble in the water and hence required suitable activation treatment, which weakens the bonds of silica and alumina with the oxygen available in the oxides of silica and alumina present in the fly ash [52, 53]. By applying the alkali fusion treatment, Shigemoto et al. have synthesized zeolite Na-X with 62% yield, while formation of zeolite Na-A was observed with the CFA enriched with the aluminum content [52]. Alkali fusion treatment is the method of choice for such kind of activation of fly ash, which extracts water soluble silicates and aluminates from the fly ash which further improve the crystallization of desired zeolites. Alkali fusion is carried out using NaOH and KOH classically [53, 54]. Chang et al. have established the method of alkali fusion followed by hydrothermal process as a general method to synthesize specific type of zeolites from the various fly ash sources [43]. They have converted the quartz and mullite phases of fly ashes into zeolites by melting fly ash with NaOH at 550°C. Their study reveals that the alkali fusion method followed by general hydrothermal method provides better crystallization then the simple hydrothermal process carried out without alkali fusion treatment [42, 43]. Murukutti and Jena have reported the alkali fusion treatment of fly ash that is carried out by Na2CO3 instead of NaOH or KOH. The use of Na2CO3 is cost effective with respect to the NaOH. The study reveal that when the fusion of fly ash is carried out with Na2CO3 or NaOH at 800°C for 2 h, fly ash transform in to the nephiline (under saturated alumino silicate), which further facilitates the crystallization of zeolites (Zeolite A at 100°C for 6 h and Zeolite X at 100°C for 8 h) through hydrothermal synthesis process, while cancrinite zeolite is formed at 100°C, with prolonged (48 h) time of hydrothermal process [53]. Park et al. have synthesized zeolite X and Y under molten condition with alkali without addition of water. The study reveal that the

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

crystallization could not be completely accomplished using the method of molten salt due to low temperature and insufficient contact of fly ash with the alkali (NaOH). Hence, the product formed using molten salt method was found to be irregular in their morphological shapes, which could not be identical match with the particular zeolites. The polycrystals of zeolites were very well developed when hydrothermal synthesis methods were used as compared to molten salt method. The study reveals that the alkali fusion method followed by the hydrothermal treatment is much reliable in order to obtain the zeolites selectively from fly ash [42, 55].

Though the alkaline fusion treatment followed by hydrothermal process improves the crystallization and yield of the zeolites, it also suffers with few disadvantages. Such as, when the oxides of silica and alumina are not in a sufficient amount, during the alkali fusion of fly ash the oxides of the elements other than Si and Al also mixed with the alkali melt products. These elements are hard to separate from the alkali melt product mix, which further carried out in the hydrothermal process and mixed up with the zeolite products, ultimately affect the quality of zeolite crystals. Thus, the alkali fusion method is suitable only when fly ash contains higher amount of oxides of silica and alumina. Moreover, alkali fusion treatment required higher energy consumption [56, 57].

The hydrothermal synthesis method suffers with the problems of longer reaction time and high energy requirement for the activation of Mullite and glassy phase of fly ash. Similarly, alkali fusion methods also required high temperature from 550 to 850°C [43, 53]. Even though these methods have been utilized extensively to produce highly crystallized and pure form of zeolites, however, these methods are not economically or environmentally benign as it required high temperature along with the longer time period [58]. Moreover, constructions of the large scale furnace and its operations are too expensive, it also generate solid waste and the handling of large quantity of liquid waste generated through this process is also a matter of concern as it contains most of the toxic metals dissolved from fly ash [59]. Hence, there is a need to find greener synthesis methods which are economically viable and environmentally benign. Few efforts have been reported via microwave and ultrasound assisted synthesis methods of CFA based zeolites.

#### *3.1.3 Microwave-assisted method*

Querol et al. [39] have introduced the microwave assisted method for the crystallization of zeolite from waste CFA. The quality, yield and types of the zeolites obtained using microwave assisted synthesis method and traditional experimental process have been found to be quite similar, but the activation time in the microwave assisted synthesis has been drastically reduced to 30 min instead of 24–48 h in traditional methods [39, 42]. Inada et al. have studied the mechanism of the microwave assisted synthesis and found that appropriate microwave radiation could increase the rate of crystallization, on the contrary, prolonged radiation time could inhibit the crystal formation of the zeolites [60]. The microwave assisted method provides uniform nucleation in the supersaturated gel solutions with rapid crystallization [56]. LTA zeolite has been synthesized from CFA by Behin and co-workers using microwave assisted method at low power of microwave radiation (100 to 300 W) within 10 to 30 min of shorter radiation time [61]. Makgabutlane et al. have observed that the zeolite crystallization from fly ash could not be accomplished successfully using microwave assisted synthesis, it needs to be combined with the conventional methods like alkali fusion and hydrothermal synthesis process [58].

#### *3.1.4 Ultrasound method*

As the microwave assisted synthesis method is under development stage and the traditional methods to extract the silica and alumina from fly ash suffers from severe disadvantages such as high energy consumption, longer reaction time and high temperature of fusion (550 to 800°C), the ultrasound method is also explored as alternative methods, recently, for the activation of fly ash and further crystallization of zeolites [56, 62, 63]. Ju et al. have observed that the ultrasonic waves helps to improve the efficiency of the extraction of silica from fly ash with compare to traditional methods [63]. Ojumu and co-worker have reported the ultrasonic treatment method for the activation of fly ash instead of high energy consuming method of alkali fusion method followed by hydrothermal synthesis method to produce zeolite A with a low energy requirement and shorter reaction time [64]. The synthesis induced by the ultrasonic waves shows rapid zeolite formation with significant reduced temperature of crystallization than the conventional hydrothermal synthesis process [62].

Chen et al. have reported the microwave and ultrasound collaborative activation method, which significantly improve the activation efficiency of silica and alumina from the fly ash with energy friendly treatment method [56]. The novel EU-12 nanozeolite has been synthesized from coal fly ash by the ultrasonication treatment method followed by hydrothermal synthesis process with 76% crystallinity [65].

#### *3.1.5 Seed assisted synthesis method*

Seed assisted synthesis method is also now a days useful to get desired zeolites using the hydrothermal synthesis process [18]. Seeding is employed in so many industrial crystallization processes to improve the quality of products. Seeding offers numerous advantages such as higher production yield, reduced induction period and contributing towards controlled particle size distribution during crystallization [37]. The details regarding the effect of seeding at the molecular level are still unexplored and need more systematic study in this area. Itabashi and coworkers have studied the effect of the zeolite seeds having different types of framework structures on the crystallization process to form targeted zeolites [66]. They have suggested a working hypothesis for the seed assisted crystallization of targeted zeolites based on the composite building units (cbu) of targeted zeolites, zeolites utilized as seeding agent and unseeded sol-gels. To validate the hypothesis, synthesis of ECR-18 has been carried out using the sol-gel composition, yielded Linde W without seed, but using the calcined ECD-18 as a seeding agent in this gel, formation of pure phase of ECD-18 is observed [66]. Similarly, zeolite CIT-1 (CON type zeolite) has been synthesized using Beta type zeolite as a promising seeding agent [67].

During the synthesis of zeolite through various steps, aging is the step where the silicates and aluminates from the sol-gel mixture converts into the precursor for the crystallization of zeolites. The crystallization of zeolite then proceeds through two steps, nucleation and crystal growth. The steps involving the formation of precursor before the starting of nucleation possess high activation energies and hence considered as the induction period. The presence of seeding agent can alter the nucleation process. The crystal of seeding zeolites act as nuclei and the crystals of targeted zeolites start to grow on the active surfaces of the seeding agent. As the crystals of products started to grow, the activation energies are considered to be lower and the growth of the crystals is faster than the nucleation [37]. The equilibrium between secondary nucleation and growth of seed crystals depends upon the quantity of gel

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

materials, nature of the system and degree of agitation. If the seed crystals with sufficient surface area which can accommodate maximum available flux of growth species by preventing the effective solution to reach high levels of super saturation. In this condition, the most of the growth of the crystals in the system will take place on the available active area of the seeded crystals. In such case, the size and rate of growth can be controlled and predicted closely by kinetic study. If the seeded crystals with reduced surface area is added to the solution, then the natural super saturation and self-nucleation will no longer be suppressed and ultimately lead to the different kind of crystal size and type than targeted zeolites [37, 68–70].

The seed assisted methods is also explored for the OSDA (Organic Structure Directing Agent) free and ultrafast synthesis of specific type of zeolites. The zeolites obtained through OSDA free seed assisted synthesis are found well crystallized with less defects within the structure of the products than the zeolites synthesized with the OSDA. This is because the seed crystal accelerate the crystal growth and there is no need of calcination to remove OSDA, which often leads to the dealumination and resulting in the creation of defective sites [37]. Liu and coworkers have reported the ultrafast seed assisted synthesis method for the synthesis of AlPO-5 and SSZ-13, the method offers rapid crystallization of zeolites AlPO-5 (1 min) and SSZ-13 (10 min). The crystallization is carried out in tubular stainless steel reactor with fast heating in preheated oil bath. This kind of reactor can be useful for mass production with continuous operation [37, 71, 72].

Different methods that are used for the synthesis of CFA-derived zeolites along with the fly ash elemental compositions are summarized in **Table 2**.






#### **Table 2.**

*Methods for the synthesis of CFA-derived zeolites with CFA composition.*

It is evident from the **Table 2** that, Zeolite Na-P, Zeolite X and Zeolite A are the most commonly synthesized zeolites from the CFA [2, 32, 41, 49, 51, 53, 56, 58, 73, 74]. It is observed that, at the lower temperature, at around, 100°C, the conversion of CFA favors the formation of Zeolite Na-P and Zeolite A crystals [2, 32, 41, 49, 51, 53, 56, 73]. While at a high temperature, at around, 160°C and a longer period of crystallization, CFA tends to form ZSM-5 crystals [21, 25, 75]. Formation of Analcime (in presence of high alumina contents in gel mixture) and Mordenite crystals are also observed at higher temperature of crystallization [18, 46, 53, 75]. When the significant amount of Na<sup>+</sup> cations are available in the gel composition, the crystallization of gel tends to form Analcime type zeolites, while at lower concentration, formation of Zeolite Na-P is observed [46, 49, 53, 75]. Zeolite A and X are crystallize even at room temperature in the presence of salt water (artificial seawater) but requires longer incubation period [2, 41]. Microwave assisted synthesis methods reduces the crystallization time for the formation of Zeolite A and Sodalite to only 10–60 min, which in conventional method takes up to few hours and sometimes up to few days [56, 58, 74]. While the use of ultrasonication during the crystallization of Nanozeolite-Na-X and EU-12 provides efficient dissolution of Si and Al within a few minutes compared to the conventional methods, thereby helping to reduce the period of hydrothermal process from few hours to only few minutes [56, 65].

#### **4. Properties and applications of fly ash-derived zeolites**

Zeolites exhibit unique properties such as higher thermal stability, high surface area and pore volume, chemical resistivity, and possess both acidic sites, Lewis and Brønsted. These properties enable zeolites to be utilized as catalysts and adsorbents in numerous industrial processes, such as cracking of petroleum crude, trans alkylation, hydro-isomerization, methylamine synthesis, and disproportionation [76]. Zeolites exhibit excellent cation exchange capacity making it capable to be utilized in the field of environmental remediation as the adsorbents and exchangers for the successful removal of pollutants such as toxic metals, nutrients and radionuclides. For example, as shown in **Table 2**, Kumar and Jena, 2022 have synthesized Zeolite NaP1 from the waste CFA, which possess good CEC (Cation Exchange Capacity), that is, 4.2 meq. NH4 + /g. They have evaluate its performance for the removal of Sr2+ and Cs2+ from the nuclear waste and found 92.5 and 39.3 mg/g adsorption capacities, respectively [46]. Similarly, Vichaphund et al. have synthesized ZSM-5-type zeolite from the CFA shows acidity values of 0.979 mmol NH4 + /g and further utilized it as catalyst for the fast pyrolysis of Jatropha waste. This catalyst shows high selectivity toward aromatic hydrocarbons formation and obtained with the yields of 97.4% [25]. The synthesized zeolites (from CFA) have been explored mainly as catalyst and sorbents. **Table 3** summarizes the properties and applications of fly ash-derived zeolites.

**Table 3** reveals that the BET surface area (SBET) of CFA-derived zeolites synthesized *via* classical methods was found to be less, which is in the range of 7.8–117 cm2 /g. The surface area of such zeolite can be improved by employing advance technique, such as ultrasonication and microwave irradiation combined with ultrasonication. It was observed that when ultrasonication method was utilized, the formation of nanozeolite Na-X (ZFH2) was observed, this nano-zeolite possesses high surface area (486 cm<sup>2</sup> /g) as shown in **Table 3** [77]. Similarly, EU-12 zeolite synthesized using sonication also shows high surface area (236 cm<sup>2</sup> /g) [65]. The combined microwave and ultrasonication technique also seems to provide high surface area zeolites, such as






#### **Table 3.**

*Properties of zeolites derived from coal fly ash (CFA) and their applications.*

LTA zeolite (10-MU-75-LTA) synthesized using combined microwaveultrasonication, was found to exhibit 442 cm<sup>2</sup> /g surface area. The ultrasonication enhances the dissolution of silica and alumina contents from the CFA into the alkaline gel mixture, which ultimately leads to the formation of the zeolites. In addition to that during the sonication, effects of acoustic cavitation along with the collapsing of microbubbles in the alkaline reaction mixture additionally enhance the rate of secondary nucleation, and quicken the mass transfer and expansion in the reactive surface area by means of fragmentation of solid crystals, which further help in reducing the time and temperature for the zeolite crystallization from CFA [77–79].

#### **5. Conclusion and future perspectives**

This review addresses the environmental problems caused due to fly ash generation and its mitigation measures *via* cost-effective and ecofriendly utilization of waste fly ash. Due to the presence of considerable amount of metal oxides, mainly SiO2 and Al2O3 in fly ash, it is widely accepted as precursor of choice for the zeolite synthesis. The information provided in this review will be used as a guiding tool for the selection of fly ash source and an appropriate synthetic route to follow for the synthesis of a particular zeolite of desired properties for its targeted applications. A brief exposure to various synthetic approaches has been discussed in this chapter, which is currently in

#### *Transformation of Waste Coal Fly Ash into Zeolites for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.108252*

practice for zeolite synthesis. In recent times, the microwave and ultrasonication methods seem promising over conventional hydrothermal methods. However, still challenges exist for their successful large-scale implementation. Further studies and research efforts are required to enhance the feasibility to transfer the technology from lab scale to industrial scale. In general, various zeolites such as ZSM-5, Zeolite A, Zeolite Na-P, Zeolite X, Zeolite Y, Analcime are synthesized from fly ash. However, the synthesis of other zeolites having high surface area, pore volume, high acidity, and high cation exchange capacity and thus having commercial market values such as zeolite β, Mordenite, MCM-22 can be explored *via* systematic study of different synthetic parameters and methods. Moreover, there is a need to develop a process, which is associated with minimum waste generation and providing maximum utilization of the elemental composition of fly ash by extracting elements other than Si and Al, separately such as Fe2O3, TiO2, trace amount of rare earth elements. Apart from these, emphases have been put in this chapter on fly ash utilization for generation of value-added materials such as zeolites and applications of fly ash-derived zeolites in the field of catalysis and environmental remediation. Future scope exists in executing large-scale application of fly ash in waste land reclamation, floriculture, and heavy metals recovery.

#### **Author details**

Henilkumar M. Lankapati<sup>1</sup> , Kalpana C. Maheria<sup>1</sup> \* and Ajay K. Dalai<sup>2</sup>

1 Department of Chemistry, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India

2 Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK, Canada

\*Address all correspondence to: kcm@chem.svnit.ac.in

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by Yongseung Yun*

With worsening climate change worldwide, the use of coal energy must be reevaluated. Lessons from highly optimized experiences in developed countries, along with innovative statistical tools, are ready to be employed in the coal energy field. Leveraging these resources can enhance efficiency, reduce waste, and promote sustainable practices in coal utilization. This book provides a comprehensive overview of the coal energy industry in the 21st century. It includes six chapters organized into three sections on the past and future of coal energy, application of statistical tools, and application technologies. Chapters address such topics as the pros and cons of coal energy, current clean coal technologies, the application of statistical tools to improve productivity and effectiveness in the coal energy industry, utilization of waste coal fly ash, and more.

Published in London, UK © 2024 IntechOpen © Sanny11 / iStock

Recent Advances for Coal Energy in the 21st Century

Recent Advances for Coal

Energy in the 21st Century

*Edited by Yongseung Yun*