Effect of Mean Specific Gravity on Combustion Characteristics of Selected High Ash Indian Coal

Abstract

The present research shows the combustion characteristics of different specific gravity fractions of high ash thermal coal. Coal with different mean specific gravity (SG_M) ranging from 1.25 to 2.0 was produced using the float–sink experiment. All SG_M coals were characterized with proximate analysis, ultimate analysis, higher heating value, ash analysis, Brunauer–Emmett–Teller analysis and Fourier-transform infrared spectroscopy (FTIR). Combustion experiments were performed with thermogravimetric analysis to identify the impact of SG_M. Experimental results inferred that ignition temperature ranged from 285 to 408 °C as SG_M varied from 1.25 to 2.0. The combustion rate of 1.25 SG_M coal was found to be the highest due to the strong or moderate presence of hydrocarbons like alkane, alkene, aldehyde and alcoholic, as observed from FTIR. Activation energy ranged from 121.81 to 54.60 kJ/ mol as SG_M of coal increased from 1.25 to 2.0. Thermodynamic analysis inferred that 1.65 SG_M coal had the highest ΔS (− 158.21 J/mol. K) and minimum ΔH (48.93 kJ/ mol), inferring ease of decomposition and higher combustion reactivity.

Introduction

Developing countries depend mainly on coal for electricity generation, as coal in such countries is an affordable, cheap, secure and efficient power source for its growing economy. Due to continuous electricity demand, coal will remain one of the prime energy sources for upcoming decades due to the minimal coal price and low operational cost of thermal power plants compared to renewable-based power plants. Coal compositions and properties vary depending on the extent of alternation or degree of coalification of the original plant material from which they were derived (Wang et al., 2022). Proximate analysis (PRA), ultimate analysis (ULA) and higher heating value (HHV) evaluation are the primary characterization methods for coal. Heat release rate is also an essential parameter for the combustor design of thermal power plants. Substantial heat release over a short period of time might damage boiler tubes and worsen slagging properties of ash, and so coal with high heat release rate and low HHV is not preferred. Similarly, coal with low heat release rate and high HHV is undesirable because it might not produce steam of the necessary quality (Liu et al., 2020). Hence, coal with the desired heat release rate and HHV is essential for thermal utilities. Coal’s heat release rate depends mainly on the chemical reactivity between coal hydrocarbons and supplied oxygen. Such combustion reactivity also depends on the surface characteristics of coal and different functional groups present as hydrocarbons, usually represented as combustible matter in coal. Fixed carbon (FC), volatile matter (VM) and ULA parameters are commonly used to describe the quality and quantity of various hydrocarbons in coal. In coal, the hydrocarbons are primarily made of hydrogen, carbon, oxygen and other minor elements, including sulfur and nitrogen. Depending on the bond strength between these elements, coal's hydrocarbons react with oxygen in different ways, and accordingly, the heat release rate varies. These coal characteristics can vary significantly among mines in the same coalfield and different seams within the same mine. Therefore, knowledge of coal's internal structure is required before burning coal in a boiler.

Thermal power stations buy coal based on their HHV. Regulations of different countries state that coal from various sources, including seams, multiple sites and imported coal, is traded on the same level without considering its natural combustion properties (Tiwari et al., 2015). The effectiveness of the combustion process for pulverized coal depends on how well coal of different specific gravity mixes with the primary air brought in from the outside. Based on coal's specific gravity and particle size, air flow rate control systems are created for this process so that coal particles properly mix with air without settling down or going outside of the reactor (Nag, 2014). It is challenging to burn other types of coal with a combustor once it has been designed for a specific type of coal. Coal from other sources having different HHV and specific gravity will be unable to produce the required heat to keep the steam quality at the proper temperature and pressure, which is vital for plant operation. A similar observation for the combustion of different specific gravity coal was made by Lv et al. (2020). They conveyed that with the variation of specific gravity, coal's HHV varies due to mineral matter variation and combustible hydrocarbon availability in coal. Fortish et al. (2000) found that the specific gravity of coal particles significantly affect coal combustion characteristics. Until now, very little work has been reported by scientists to establish directly the correlation between coal specific gravity or ash with coal combustion behavior. Wu et al. (2019) found that the specific gravity of coal is critical for size segregations in pulverized coal combustion. Zhang et al. (2015) found that functional groups, such as aliphatic alkane, alkene, aromaticity, alcoholic groups, phenolic groups, in coal varies with change in coal rank. As a consequence, the combustion performance of coal fluctuates significantly based on their rank. Cao et al. (2015) observed that the availability of functional groups changes with coal particle size. Shen et al. (2018) reported combustion characteristics and flu gas composition varied significantly with change in coal’s functional groups during combustion and pyrolysis. Song et al. (2017) studied coal combustion characteristics using TG–FTIR (thermogravimetry—Fourier-transform infrared spectroscopy). They observed that CH₄ and CO₂ release varied largely with change in the presence of aliphatic chain and carboxyl groups. Sarkar et al. (2013) identified the combustion characteristics of coals using TGA (thermogravimetric analysis) and reported that high ash coal has inferior ignition temperature (T_IN), peak temperature (T_PK) and burnout temperature (T_FL) than low ash coal. Using TGA, Biswas et al. (2006) investigated the combustion behavior of two coal of same rank but with different ash content. They reported that high ash coal has poor T_PK and T_FL compared to low ash coal. Banerjee et al. (2016) estimated the variation in combustible materials, mineral compositions and ash fusibility of four coal samples collected from the different seams of the same coal mine. They found considerable variation in FC and VM with depth of seam. Mineral compositions, such as SiO₂, Al₂O₃, CaO, TiO₂ and Fe₂O₃, are almost identical, and all coals' slagging and fouling nature are on the lower side. Kumari et al. (2016) investigated the role of mineral composition during the use of coal in thermal utilities and reported that SiO₂ is the predominant mineral in Indian coal. Saini and Srivastava (2017) analyzed slagging and fouling behavior of different specific gravity fractions of coal. They found that slagging and fouling characteristics are higher for heavier specific gravity coal fractions due to high ash content and alkaline minerals. Yu et al. (2020) analyzed the impact of surface porosity on the combustion characteristics of coal. They conveyed that pore provides a good passage for O₂ to the coal surface, which improves combustion performance. Mishra (2022) found that exposure to greater surface area leads to higher oxygen diffusion into coal, which increases combustibility. Wang et al. (2018) studied the role of surface morphology on coal combustion characteristics using FESEM (field emission scanning electron microscopy) and TGA. They found that coal has combustion difficulty and requires higher activation energy due to the inferior texture of coal particles.

Earlier reports by other scientists infer that coal combustion is a sophisticated phenomenon, affected significantly by the surface properties of coal and hydrocarbons, functional groups, and inorganic mineral matter available in coal. The coal conversion rate is generally modified by altering turbulence in the combustion chamber due to the large variation in the specific gravity of coal with mineral matter quantity and composition (Gupta, 2007). Hence, a relation between coal properties and specific gravity associated with coal combustion characteristics must be established to utilize coal effectively. Generally, power plants have only PRA and HHV data available, whereas various important characteristics such as functional groups, surface characteristics, hydrocarbon types and mineral matter composition for coal are unavailable. Due to the unavailability of such crucial information, most thermal utilities increase the coal feed rate in the burner to maintain the required heat release rate. As a result, unburned coal particles and combustible gases increase in the exhaust gas. Thus, characterization such as PRA, ULA, FESEM, FTIR, ash analysis and Brunauer–Emmett–Teller (BET) analysis is essential to identify the effect of combustible and non-combustible matter, surface texture, different functional groups and mineral compositions on the combustion behavior of coal particles with different densities. Apart from those, analyses of ash slagging characteristics, enthalpy, Gibbs energy and entropy during combustion are also necessary to identify the variation of endothermicity, decomposition difficulty and nature of the reaction during the combustion of different specific gravity coal. Studies on this issues are limited until now. Hence, the major objective of this work was to develop a relation between coal’s combustion characteristics and its specific gravity or ash in order to optimize coal selection for efficient combustion characteristics in thermal power plants.

To achieve the objectives of the present research, coal of different specific gravity fractions represented by mean specific gravity (SG_M) ranging from 1.25 to 2.0 were prepared using the float–sink separation method. Prepared coals were characterized by PRA, ULA and HHV. Different hydrocarbons present as functional groups in coal were identified with FTIR. FESEM was done to identify the surface structure. Simultaneously, BET analysis was performed to explore the impact of surface porosity on coal combustion characteristics. Combustion experiments of coal with different SG_M were performed using TGA to extract critical combustion characteristics parameters. The heat release, coal consumption and ash generation rates with variation of coal SG_M were also analyzed. Further, kinetic and thermodynamic parameters were determined to get more insight into the combustion reaction.

Experimental Methodology

Sub-bituminous coal of about 150 kg was collected from the Kajora mines, Eastern Coalfields Limited, West Bengal, India. The coal samples were brought in air-tight plastic bags to the institute's coal preparation laboratory within two days of collection. After that, the entire coal samples were crushed to − 50 + 0.5 mm using a conical crusher, jaw crusher and roll crusher. Later, coal particles produced by crushing were thoroughly mixed to obtain a uniform mixture of coal. The float–sink experiments were performed by the blending of organic liquids such as bromoform (specific gravity: 2.90), trichloroethylene (specific gravity: 1.45) and kerosene (specific gravity: 0.80) in the required volume proportion to prepare specific gravity solutions ranging from 1.25 to 1.90. The specific gravity of all fractions of the solution was estimated using a hydrometer. SG_M between 1.25 and 2.0 was estimated by the arithmetic mean of each specific gravity fraction of coal. After float–sink experiment, coal of each specific gravity fraction was cleaned with tap water and then dried. About 300 gm of prototypical coal samples of − 72 mesh sizes were made by following the size decrement and sampling as per the standard procedure (Cai et al., 2021). All specific gravity fractions of coal were characterized with different characterization methods, namely, PRA (A.S.T.M D3173); HHV analysis with bomb calorimeter (Make: LECO; Model: AC 350); ULA with CHNS analyzer (Make: Vario; Model: III); BET analysis (Make: Micromeritics Instrument Corporation; Model: 3FLEX 3500); surface morphology with FESEM (Make: Supra’55; Model: Mono CL4); XRF (Make: Burker; Model: S8 Tiger). Combustion experiments were done using TGA (Make: NETZSCH; Model: STA449 F3 Jupiter). For the combustion experiment, about 15–20 mg of coal was kept in TGA crucible and was non-isothermally heated at 10 °C/min in the presence of pure O₂ with a 60 ml/min flow rate. Using TGA data, a number of combustion associated parameters, namely, initiation temperature $({\text{T}}_{{\text{IN }}})$, burnout temperature $({\text{T}}_{{{\text{FL}}}})$, maximum temperature $({\text{T}}_{{{\text{PK}}}})$ and maximum combustion rate $({\text{DTG}}_{{{\text{peak}}}})$(mass %/min) were calculated (Aich et al., 2019). After that, important performance indices, namely, initiation index (I_IN), characteristics index (C_IN), burnout index (B_IN) and heat intensity index (H_IN) were determined using the following equations (Aich et al., 2019; Chakraborty et al., 2021):

$${\text{I}}_{{{\text{IN}}}} = \frac{{{\text{DTG}}_{{{\text{peak}}}} }}{{{\text{t}}_{{{\text{IN}}}} \times {\text{t}}_{{{\text{PK}}}} }}$$

(1)

$${\text{C}}_{{{\text{IN}}}} = \frac{{{\text{DTG}}_{{{\text{peak}}}} { } \times {\text{ DTG}}_{{{\text{mean}}}} }}{{{\text{T}}_{{{\text{IN}}}}^{2} \times {\text{T}}_{{{\text{FL}}}} }}$$

(2)

$${\text{B}}_{{{\text{IN}}}} = \frac{{{\text{DTG}}_{{{\text{peak}}}} }}{{{\text{t}}_{{{\text{PK}}}} \times {\text{t}}_{{{\text{FL}}}} \times \Delta {\text{t}}_{1/2} }}$$

(3)

$${\text{H}}_{{{\text{IN}}}} = {\text{T}}_{{{\text{PK}}}} \times {\text{ln}}\left( {\frac{{{\Delta T}_{1/2} }}{{{\text{DTG}}_{{{\text{Peak}}}} }}} \right) \times 10^{ - 3}$$

(4)

where ${\text{t}}_{{{\text{IN}}}} ,{\text{ t}}_{{{\text{PK}}}}$, ${\text{t}}_{{{\text{FL}}}}$ are times corresponding to ${\text{T}}_{{\text{IN }}}$, ${\text{T}}_{{{\text{PK}}}} { }$, ${\text{T}}_{{{\text{FL}}}}$, while $\Delta {\text{t}}_{1/2}$ is time associated with ${\text{DTG}}/{\text{DTG}}_{{{\text{peak}}}} = 0.5$, ${\text{DTG}}_{{{\text{mean}}}}$ is the average combustion rate during the entire reaction. Activation energy (${\text{E}}_{{\text{C}}}$) and pre-exponential factor (f) during combustion were estimated by the Coats–Redfern (CR) method (Nyoni et al., 2020; Zou et al., 2022), thus:

$${\text{ln}}\frac{{ - {\text{ln}}\left( {1 - \varphi } \right)}}{{{\text{T}}^{2} }} = \ln \left( {\frac{{f{ } \times {\text{ R}}}}{{\lambda { } \times {\text{ E}}_{C} }}\left[ {1 - \left( {\frac{{2{\text{R }} \times {\text{ T}}}}{{E_{C} }}} \right)} \right]} \right) - \frac{{{\text{E}}_{C} }}{{{\text{R }} \times {\text{ T}}}}$$

(5)

where φ is fuel conversion, λ is heating rate, E_C is activation energy in kJ/mol, f is pre-exponential factor in sec^-1, R is the universal gas constant (8.314 J/K mol), T is temperature (K). Change in thermodynamic parameters, namely, enthalpy $(\Delta {\text{H}})$, Gibbs free energy (ΔG) and entropy (ΔS) were estimated respectively as (Mishra et al., 2020; Merdun & Laouge, 2021):

$$\Delta {\text{H}} = {\text{ E}}_{{\text{C}}} - {\text{ R }} \times {\text{ T}}_{{{\text{PK}}}}$$

(6)

$$\Delta G = E_{C} + R \times T_{PK} \times \ln \left( {\frac{{\Psi \times T_{PK} }}{\varepsilon \times f}} \right)$$

(7)

$$\Delta {\text{S}} = \frac{{\Delta {\text{H}} - { }\Delta {\text{G}}}}{{{\text{T}}_{{{\text{PK}}}} }}$$

(8)

where ${\Psi }$ is Boltzmann constant (1.3806 × 10⁻²³) and $\varepsilon$ is the Planck constant (6.626 × 10^-34).

Results and Discussions

Impact of SG_M on Coal Properties

The variations in proximate and ultimate analysis, HHV and fuel ratio based on dry ash free basis along with weight distribution corresponding to the head sample and each SG_M of coal are shown in Table 1. From Table 1, head coal sample had lower ash (20.21%) but higher VM (46.30%) and FC (77.20%) compared to typical high ash Indian coal (ash: 29–40%; VM: 20–30%; FC: 23–53%) (Raghuvanshi et al., 2022). Similarly, C (77.20%), H (8.24%), S (2.67%) and N (2.05%) were on the higher side. From Table 1, it is observed that, as SG_M of coal increases from 1.25 to 2.0, ash deteriorates from 2.41 to 76.36%, VM changes from 46.30 to 66.70% and FC declines from 66.56 to 33.35%. Such changes can be described by the fact that FC’s specific gravity is lighter than mineral matter's. Thus, with rise in coal SG_M, mineral matter represented by ash content rises while carbonaceous content reduces, FC declines, while VM also remarkably changes. Such observations are further verified with the ULA, indicating that C deteriorates from 79.64 to 53.65% and H changes from 5.33 to 6.48% as SG_M rise from 1.25 to 2.0. Coal with 1.25 SG_M had the highest fuel ratio 1.34. Fuel ratio declined from 1.34 to 0.5 as SG_M rose from 1.25 to 2.0. Such reduction in fuel ratio with SG_M can be attributed to substantial reduction in FC and rise in VM of coal. Table 1 shows that HHV of 1.25 SG_M coal was maximum (9606 kcal/kg). Further, HHV reduced from 9606 to 6577 kcal/kg as coal SG_M rose from 1.25 to 2.0. Such investigations are consistent with the decline of C and O contents in coal as SG_M increases, reducing coal's combustibles and oxidative environment. Therefore, availability of energy density in coal declines as SG_M increases.

Table 1 Fluctuations in physical–chemical properties, HHV and fuel ratio^* of coal with SG_M

Impact of SG_M on FTIR

Fluctuations in functional groups with the change in coal SG_M were investigated with FTIR. Figure 1a–b depicts functional groups and corresponding peaks extracted from FTIR and results are summarized in Table 2. Figure 1a–b and Table 2 illustrate notable deviations in the bond intensity of functional groups for different wavelengths with the variation in SG_M of coals. Peaks in 3700–3600 cm⁻¹ associated with O–H bond for alcoholic functional groups were absent in 1.25–1.375 SG_M coal, weak in 1.425 and 1.475 SG_M coal, while strong for 1.55 to 2.0 SG_M coal (Chakravarty et al., 2020). This infers that hydrocarbon-rich alcoholic functional groups increase with rise in coal SG_M. Peaks between 3450 and 3350 cm^-1 related with N–H group for prime amines were stronger for 1.25–1.375 SG_M coal, moderate for 1.425 and 1.475 SG_M coal and mild for 1.55–2.0 SG_M coal (Odeh, 2015). Such observation implies that prime amines enriched hydrocarbons reduce in coal when coal SG_M increases. Similarly, peak of C–H group associated with aliphatic CH₃ and aliphatic CH₂ asymmetric stretching vibration between 3000–2800 cm^-1 wavelength were found mild for 1.65–2.0 SG_M coal, moderate for 1.475–1.55 SG_M coal and strong for 1.25–1.425 SG_M coal (Xuguang 2005; Chakravarty et al., 2020). This shows that, with increase in SG_M, aliphatic rich hydrocarbon reduces. Similar examinations were also seen for aromatic nucleus associated with C=C stretching at 1650–1500 cm^-1 and were stronger for 1.25–1.375 SG_M coal and moderate for 1.425–2.0 SG_M coal (Mishra et al., 2016a). The above finding infers that all SG fractions of coal contain aromatic C = C ring, decreasing with increase in coal SG_M. Peaks around 1500–1400 cm^-1 for aliphatic chain of C–H band associated with alkanes and aldehyde were moderate for 1.25–1.425 SG_M coal, weak for 1.475–1.55 SG_M coal and absent for 1.65–2.0 SG_M coal (Odeh, 2015; Mishra et al., 2016a). The C–O–C stretching corresponding to anhydride hydrocarbons within 1050–1000 cm⁻¹ was present in all SG_M coal, signifying all coals have anhydride compounds (Wang et al., 2016).

Figure 1

(a–b) Variation in functional groups with different SG_M coal.

Table 2 Functional group fluctuations with SG_M coal

Peaks at 970–900 cm⁻¹ for C=C stretching related to alkenes were absent for 1.25–1.375 SG_M coal but were available with moderate to strong intensity for 1.425–2.0 SG_M coal; inferring lighter SG_M coal has unavailability of C=C bond for alkene hydrocarbons. Peaks at 800–750 cm⁻¹ associated with C=C stretching of alkene (tri-substituted) hydrocarbons were very strong in 1.55–2.0 SGM coal and present moderate or weak peaks for all other SG_M coal. Peaks at 550–450 cm⁻¹ associated with C–I peak corresponding to halogen compounds were weak for 1.25–1.275 SG_M, moderate for 1.325–1.375 SG_M and strong for 1.425–2.0 SG_M coal. Overall, results obtained from FTIR showed that O–H bonding related to alcoholic hydrocarbons, C=C stretching for tri-substituted alkene and C–H stretching due to aldehyde are weakly present or nonexistent in 1.25–1.375 SG_M coal although they are strongly present in 1.55–2.0 SG_M coal and moderately or mildly present in 1.425 and 1.475 SG_M coal. Thus, different SG_M coal consists of various hydrocarbons and will impact coal combustion performance accordingly.

Impact of SG_M on Surface Morphology and Pore Characteristics

Figure 2 illustrates the variation in surface morphology with coal SG_M. Figure 2 shows 1.25 SGM coal has more longitudinal cracks than heavier density fraction coals. Such cracks promote the diffusion of gases and consequently lower endothermic energy is required for the conversion of various functional groups, such as C=C tri-substituted alkenes (800–750 cm⁻¹), alkene (970–900 cm⁻¹), aliphatic chains of CH bending such as aldehyde and alkane (1500–1400 cm⁻¹), aliphatic-CH₂ and -CH₃ stretching (3000–2800 cm⁻¹) and alcoholic/prime amine group (3400–3350 cm⁻¹), of 1.25 SG_M coal into various intermediate products and finally to H₂O and CO₂ (Wang et al., 2016; Kumar and Nandi 2021). Coals with 1.275 and 1.325 SG_M show good porous texture, promoting their reactivity, while heavier SGM coals have less porous or non-porous texture. Such inferior porous morphology signifying larger decomposition energy would be required for the conversion of heavier SG_M coals into end products, and, as a result, the combustion of such coals in power plants are challenging, as the use of such coals may damage the boiler tubes of different sections due to significant deviation in heat received by boiler tubes from the designed values.

Figure 2

FESEM images of different SG_M coal.

BET analysis (adsorption isotherm) was carried out to gain better insights to coal surface texture with variation in coal SG_M. Pore characteristic parameters such as mean pore size, specific surface area and pore volume with varying coal SG_M were obtained and shown in Table 3. Table 3 shows that the mean pore size of coal fluctuated significantly as coal specific gravity varied. Lighter SG_M coals, namely 1.25, 1.275, 1.325, 1.375 and 1.475 showed relatively larger pore sizes than heavier SG_M coal such as 1.425, 1.55, 1.65, 1.75 and 1.85. Such variations across different SG_M coals are attributed to availability or unavailability of different kinds of functional groups, such as C=C tri-substituted alkenes, alkene, aromatic aldehyde/alkane, aliphatic-CH₂/-CH₃ stretching and alcoholic/prime amine group, and variation in distributions of combustible and mineral matters across different SG_M of coal (Sampath et al. 2020; Fang et al., 2022). Overall, it can be concluded that 1.425 SG_M coal had the smallest pore size of 4.46 nm, and the highest pore volume of 55.68 × 10⁻⁴ (cm³/g) and surface area of 256.36 m²/g. The presence of such higher surface area and pore volumes in 1.425 SG_M coal will ease the diffusion of gases and consequently, lower decomposition energy is required for the conversion of functional groups into various intermediate products and finally to H₂O and CO₂ (Wang et al., 2016; Kumar and Nandi 2021). Thus, it is expected that 1.425 SG_M coal will ignite easily compared to other SG_M coal during combustion. Table 3 shows that the specific surface area increased with SG_M of coal. Such improvement in surface area may be attributed to the rise of inorganic mineral oxides such as MgO, Fe₂O₃, CaO and Na₂O (as shown in Table 4) in coal (Sun et al., 2013). As inorganic matter constitutes different inorganic salts, they are relatively more porous than combustibles (Ronsse et al., 2013). Thus, lighter SG_M coal has lower surface area due to low inorganic constraints, and with increase in SG_M from 1.425 to heavier density fractions surface area reduces. The highest pore volumes and surface area for 1.425 SG_M coal inferred the optimum combinations of inorganic matter and combustibles composition to yield highly porous coal.

Table 3 Variation of mean pore size, specific surface area and total pore volume with the variation of coal SG_M

Table 4 Variation in ash constituents (in mass%) with SG_M of coal

XRF of Coal

Fluctuations in inorganic oxides of different SG_M coal’s ash are obtained from XRF and are summarized in Table 4. Table 4 shows that 1.25 SG_M coal had minimum content of SiO₂ (46.72%) and Al₂O₃ (24.30%) compared to other heavier SG_M coals. SiO₂ and Al₂O₃ are alkaline with four coordination numbers and are major oxides of concern with respect to the slagging and fouling properties of coal. Fe₂O₃ and CaO are non-fluxing alkaline oxides with six co-ordination numbers. MgO is another alkaline earth metal oxide present in coal. Coal with 1.85 SG_M had a higher amount of MgO, while 2.0 SG_M coal contained higher quantities of Fe₂O₃ (10.12%) and CaO (14.97%). MnO, Na₂O, K₂O, P₂O₅ and Cr₂O₃ are basic oxides, and were in smaller quantities in all coal samples. The availability of different alkaline and basic oxides significantly affects the coal combustion properties by modifying the ash thickness characteristics as well as slagging and fouling tendencies. Slagging and fouling parameters such as base acid ratio (R_BA), fouling factor (F_IF) and silica ratio (S_RT) were estimated using inorganic oxides based on Eqs. (9)–(11) (García et al., 2015; Mishra et al. 2016b) and are summarized in Table 5.

$${\text{R}}_{{{\text{BA}}}} = { }\frac{{{\text{K}}_{2} {\text{O}} + {\text{ Na}}_{2} {\text{O}} + {\text{ Fe}}_{2} {\text{O}}_{3} + {\text{MgO}} + {\text{CaO}}}}{{{\text{Al}}_{2} {\text{O}}_{3} + {\text{ SiO}}_{2} + {\text{ TiO}}_{2} }}$$

(9)

$${\text{F}}_{{{\text{IF}}}} = {\text{ R}}_{{{\text{BA}}}} \times \left( {{\text{K}}_{2} {\text{O}} + {\text{ Na}}_{2} {\text{O}}} \right)$$

(10)

$${\text{S}}_{{{\text{RT}}}} = { }\frac{{{\text{SiO}}_{2} }}{{{\text{SiO}}_{2} + {\text{CaO}} + {\text{MgO}} + {\text{Fe}}_{2} {\text{O}}_{3} }}$$

(11)

where R_BA infers the ash slagging nature of coal, with higher value of R_BA signifies higher slag formation nature on the boiler tubes. Theoretically, R_BA value < 0.5 signifies mild ash deposition, 0.5–1 moderate ash deposition while R_BA > 1 indicates high deposition properties (García et al., 2015). F_IF ≤ 0.6 signifies mild fouling characteristics, while 0.60–40 shows high fouling nature of coal (García et al., 2015). S_RT is an indicator of slagging performance, and good coal having S_RT > 0.78 infers hard to fuse (Mishra et al., 2016b).

Table 5 Variation in slagging and fouling parameters with SGM of coal

Table 5 presents the variations in R_BA, F_IF and S_RT for different SG_M coals. Table 5 signifies that, as coal SG_M increased from 1.25 to 2.0, R_BA increased from 0.19 to 1.19 and F_IF increased from 0.35 to 0.97, signifying that heavier SG_M coals have higher slagging and fouling tendency. Similarly, S_RT values of 1.85–2.0 SG_M coal were very inferior compared to the permissible limit (< 0.78), signifying higher melting nature of heavier SG_M coals. Overall, based on ash characterization, it can be concluded that 1.25 to 1.75 SG_M coals can be utilized in thermal power plants.

Impact of SG_M on Combustion Characteristics

Combustion analysis for different SG_M coal fractions was done under the oxygen (O₂) atmosphere using TGA–DTG investigation. Variations in mass loss (TGA) and mass loss rate (DTG) profiles with SG_M of coal are shown in Figure 3. Figure 3a infers that all coal samples exhibit distinguished mass loss profiles according to functional groups, surface properties, hydrocarbons and mineral compositions present in individual coals. About 2–3% mass loss happened within 30–150 °C due to moisture evaporation from the coal texture. Later, some mass gain in 150–300 °C is seen, related to oxygen adsorption into coal porous surface texture. Further, a significant mass loss was found for all SG_M coal fractions, inferring the initiation of combustion. Such significant mass loss is due to the release of lighter hydrocarbons and organic volatile matter due to the heating of coal, delivering the requisite energy to ignite the solid carbon available in the sample (Kumar and Nandi, 2022). Within 450–500 °C, a sharp mass loss was found, signifying that the rate of combustion was at its maximum in this temperature region. After that, the combustion rate was almost negligible, signifying the completion of the combustion process due to the total burnout of carbonaceous material available in coal. The DTG curve shown in Figure 3b comprehensively describes such mass loss profiles. The number of coal combustion characteristics parameters, such as ${\text{T}}_{{\text{IN }}}$, ${\text{T}}_{{{\text{PK}}}}$, ${\text{T}}_{{{\text{FL}}}}$ and ${\text{DTG}}_{{{\text{peak}}}}$ were extracted from TGA and are summarized in Table 6. Table 6 and Figure 3b show that the mass loss pattern of specific coal samples can be attributed to the combustion behavior of that coal samples. Coal with 1.25 SG_M had the maximum mass loss rate (38.5 mass%/min) compared to the other heavier SG_M coals. This infers that 1.25 SG_M coal is most favorable for thermal utilities with superior combustion rate that can increase the steam generation rate due to maximum heat release rate. Such superior rate of mass loss of 1.25 SG_M coal can be linked with the availability and non-availability of different functional groups in coal as discussed in section Impact of SGM on FTIR, inferring that 1.25 SG_M coal has all kinds of combustible functional groups. However, the combustion rate is inferior for heavier density coals which can be attributed to weak peak or absence of different functional groups such as C=C tri-substituted alkenes (800–750 cm⁻¹), alkene (970–900 cm⁻¹), aliphatic chains of CH bending such as aldehyde and alkane (1500–1400 cm⁻¹), aliphatic–CH₂ and –CH₃ stretching (3000–2800 cm⁻¹) and alcoholic/prime amine group (3400–3350 cm⁻¹) as found from Table 2. These findings infer that coal consists of tri-substituted alkenes, aldehyde, alkane, alcoholic and prime amine groups with good combustion properties and exhibiting a better combustion rate. These functional groups in coal might be available as FC and VM, which play a crucial role during combustion. From Table 6, it is found that when coal SG_M progressed from 1.275 to 2.0, ${\text{T}}_{{\text{IN }}}$ rose from 285 to 408 °C, T_PK ranged from 398 to 425 °C, T_FL varied from 469 to 497 °C. All such temperatures are on the higher side for 1.25 SG_M coal. Such fluctuations may be attributed to the higher oxygen availability in 1.275 SG_M coal (Table 1). Later, it was seen that T_IN reduced with progress in SG_M of coal. Such fluctuations of T_IN are due to a decrease of VM with rise in coal SG_M as VM promotes ease of ignition. Overall, it can be concluded that heavier SG_M coal is more difficult to ignite. The impact of SG_M of coal on different performance indices, namely, I_IN, C_IN, B_IN and H_IN are evaluated using Eqs. (1)–(4), and are summarized in Table 6. Ignition index (I_IN) value should be higher as it infers the ignition properties of coal (Aich et al., 2019). Characteristics index (C_IN) should be higher as it indicates the combustion behavior of coal (Xinjie et al., 2021). Burnout index (B_IN) shows the reactivity state between coal and air/oxygen. A higher B_IN value indicates improved combustion reactivity and adequate energy release to sustain the combustion process (Kumar and Nandi, 2022). However, the heat intensity index (H_IN) represents combustion stability and the smaller value represents superior combustion characteristics (Aich et al., 2019). Table 6 shows that I_IN reduced from 38.1 to 0.9 mass/min³ as SG_M of coal increased from 1.25 to 2.0, indicating ignition difficulty increases with increase in SG_M. Such ignition problems may be due to reduced hydrogen and VM content in coal for higher SG_M coals. Similarly, C_IN degraded from 101.1 to 1.9 mass²/min^2 °C³ as SG_M of coal increased from 1.25 to 2.0, representing difficulty in combustion with increase in SG_M. Further, with the progression of SGM from 1.25 to 2.0, B_IN reduced from 76.4 to 2.3 mass/min⁴, signifying that high SG_M coal requires more time for complete combustion. Overall, it can be summarized that 1.25–1.55 SG_M coals show better combustion properties compared to other heavier SG_M coals. H_IN values enhanced from 0.9 to 2.4 °C, with the highest value of 2.4 °C for 2.0 SG_M coal. This indicates that 2.0 SG_M coal has poor combustion performance compared to other SG_M coals. Overall, the analysis of performance indices and burning profile parameters convey that the combustion characteristics of 1.325–1.55 SG_M coal are superior to other SG_M coals.

Figure 3

TGA–DTG profiles for combustion of different SG_M coal.

Table 6 Changes in combustion profile parameters with the variation of coal SG_M

Impact of SG_M on Coal Utilization

Coal coming to thermal utilities must have sufficient heat release rates to keep the combustor temperature at the optimum level. In this context, heat release rate (R_H, kcal/min) and minimum burning duration (D_M, min) for a unit quantity of coal were calculated based on HHV and maximum combustion rate (DTG_Peak) of coal using the following equations (Kumar and Nandi, 2022):

$${\text{R}}_{{\text{H}}} = \frac{{{\text{HHV}} \times {\text{ DTG}}_{{{\text{Peak}}}} }}{100} = \frac{kcal}{{kg}} \times \frac{kg}{{100 \times min}} = k{\text{cal}}/{\text{min}}$$

(12)

$${\text{D}}_{{\text{M}}} = \frac{1gm}{{\frac{{DTG_{Peak} }}{100} \times \frac{gm}{{min}}}} = \frac{100}{{DTG_{Peak} }}{\text{min}}$$

(13)

In the pulverized combustion chamber, the entire range of coal particles have an equal duration to burn. This burning duration depends on coal's turbulence and settling velocity in the combustion chamber. All the coal particles must be burnt before coming outside the combustor as a bottom ash. With variation of chemical and physical properties of coal, the particles might not burn out in a timely manner. In such conditions, D_M could act as a key parameter in determining how effectively coal burns. Theoretically, D_M is the minimum time duration for total burnout of any coal particles. Similarly, R_H is the maximal heat release rate associated with produced steam quality. Figure 4 represents the fluctuations of D_M and R_H with the variation in SG_M of coal. It is observed from Figure 4 that, with variation in SG_M of coal from 1.25 to 2.0, D_M ranged from 3 to 75 min while R_H ranged from 3292 to 19.1 kcal/min. Coal with 1.25 SG_M showed the maximum R_H (3292 kcal/min) and minimum D_M (3 min). Similarly, a minimum R_H of 19.1 kcal/min was seen for 2.0 SG_M with maximum D_M (75 min). This observation infers that heavier SG_M coal requires longer duration for complete combustion than lighter SG_M coals. Such fluctuation in R_H and D_M are attributed to the increase in mineral matter and decline in combustible matter present in coal with increase in SG_M. Higher ash thickness associated with increased mineral matter creates obstacles for the diffusion of O₂, CO₂, etc. gases, and consequently, the combustion rate deteriorates.

Figure 4

Fluctuations of heat release rate and burning duration with coal SG_M.

Thus, selecting coal with the optimum SG_M to run a combustor efficiently is very important. Overall, it can be concluded that coals with 1.25 to 1.55 SG_M infer better heat release rate with lower combustion duration than 2.0 SG_M coal.

Impact of SG_M on Coal Feed Rate and Ash Generation Rate

As discussed in the previous section (Impact of SGM on Coal Utilization), the rate of heat release (R_H, kcal/min) substantially fluctuated with SG_M of coal and depended on HHV, ash content and DTG_Peak. Due to fluctuations in ash, HHV, combustion rate and R_H, the required coal feed and corresponding ash generation rate for constant energy output also significantly deviated as thermal power plants are required to combust different amounts of coal to sustain uniform electricity production (Kumar and Nandi, 2022). Coal feed rate (${\text{W}}_{{{\text{Fuel}}}}$) and ash generation rate $\left( {{\text{W}}_{{{\text{ash}}}} } \right)$ for 10000 kcal heat generation were calculated respectively using the following equations (Kumar and Nandi, 2022):

$${\text{W}}_{{{\text{Fuel}}}} = { }\frac{10000}{{{\text{HHV}}}}$$

(14)

$${\text{W}}_{{{\text{ash}}}} = { }\frac{{{\text{W}}_{{{\text{Fuel}}}} \times {\text{Ash }}\left( {\text{\% }} \right)}}{100}$$

(15)

Variations in ${\text{W}}_{{{\text{Fuel}}}}$ and ${\text{W}}_{{{\text{ash}}}}$ with variation in SG_M of coal is shown in Figure 5. Figure 5 shows that coal consumption increased from 1.17 to 7.03 kg as coal SG_M rose from 1.25 to 2.0. This rise of coal consumption is due to the reduction of HHV with SG_M of coal. Ash generation rate also significantly increased from 0.03 to 5.37 kg as SG_M of coal increased.

Figure 5

Fluctuations of ${\text{W}}_{{{\text{Fuel}}}}$ and ${\text{W}}_{{{\text{ash}}}}$ with coal SG_M.

Such observations for thermal power plants infer that it is important to use coal with needed combustion characteristics with lower ash content and higher HHV. Based on the lower coal consumption and ash generation rates, 1.25 to 1.55 SG_M coal should be used in thermal power plants.

Impact of SG_M on Coal Combustion Kinetics

Fluctuation of activation energy (E_C) and pre-exponential factor (f) for different SG_M coal were determined using Eq. (5) and are shown in Figure 6. From Figure 6a, it is clear that, with increase in SG_M from 1.25 to 2.0, E_C reduced from 121.81 to 54.60 kJ/mol. The minimum E_C was found for 1.65 SG_M coal and maximum E_C for 1.25 SG_M coal. Such fluctuation in E_C with SG_M may be attributed to changes in surface characteristics and availability of functional groups. For 1.65 SG_M coal, almost all kinds of functional groups are available, excluding only the aldehyde group at 1500–1400 cm⁻¹. For 1.25 SG_M coal, surface area and pore volume were inferior, and functional groups such as tri-substituted alkenes (800–750 cm⁻¹), alkenes (970–900 cm⁻¹), aliphatic chains of C–H bending (1500–1400 cm⁻¹) and alcoholic group (3700–3650 cm⁻¹) are either weakly present or absent as mentioned in Table 2. Figure 6b shows that f increased from 1.96 × 10⁴ sec⁻¹ to 21.23 × 10⁴ sec⁻¹ as SG_M progressed from 1.25 to 2.0, inferring the lowest combustion reactivity of lighter SG_M coal compared to other heavier SG_M coal. Such fluctuation in f with SG_M is due to deviation of different functional groups availability in individual coal. However, the highest f was observed for 1.65 SG_M coal and the lowest for 1.25 SG_M coal.

Figure 6

Fluctuations of (a) E_C and (b) f with coal SG_M.

For coal with 1.65 SG_M, surface properties were good and almost all functional groups were available, excluding only the aldehyde group at 1500–1400 cm⁻¹. For coal with 1.25 SG_M, surface characteristics were inferior, and most of the functional groups, such as alkenes, tri-substituted alkenes, aldehyde and alcoholic groups were absent or weakly present. Such observations infer that, for better combustion characteristics with minimum E_C and maximum f, almost all of the functional groups should be available in coal.

Impact of SG_M on Thermodynamic Parameters

Variations in enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS) with coal SG_M were evaluated using Eqs. (6)–(8) and are shown in Figure 7. ΔH infers reaction behavior (exothermic/endothermic) during coal combustion. The higher ΔH value signifies more heat input is needed during the coal combustion (Mishra and Mohanty, 2020). From Figure 7a it can be noticed that, with increase in SG_M from 1.25 to 2.0, ΔH reduced from 116.80 to 48.93 kJ/mol. A lower ΔH value was seen for 1.65 SG_M coal, while the highest ΔH for 1.25 SG_M coal, signifying that less endothermic energy is needed for 1.65 SGM coal than other coals. Such variations in ΔH with SG_M may be due to variations in functional groups' availability. For coal with 1.65 SG_M, most functional groups were available, excluding only the aliphatic chain of C–H bending between 1500 and 1400 cm⁻¹ wavelength. Whereas for coal with 1.25 SG_M, large number of functional groups such as C=C tri-substituted alkenes (800–750 cm⁻¹), alkene (970–900 cm⁻¹), aldehyde (1500–1400 cm⁻¹), alkane (3000–2800 cm⁻¹) and alcoholic/prime amine group (3400–3350 cm^-1) were either weakly present or absent as shown in Table 2. ΔG signifies the decomposition difficulty of coal combustion. The higher ΔG value indicates that more decomposition energy is needed during the reaction (Dhyani et al., 2017). Figure 7b shows that ΔG value of 1.25 SG_M coal was maximum (238.54 kJ/mol) compared to other coals, signifying more decomposition energy is needed for 1.25 SG_M coal. Such observation may be due to lower surface characteristics and unavailability of combustible functional groups such as tri-substituted alkenes, aldehyde, alcoholic and prime amine groups. However, ΔG value was the lowest for 1.65 SG_M coal, which may be due to good surface characteristics and the presence of almost all functional groups except the aldehyde group. ΔS value should be higher as it represents coal combustion reactivity (Aprianti et al., 2023). From Figure 7c, ΔS value was lowest (− 177.98 J/mol. K) for 1.25 SG_M coal and highest (− 158.22 J/mol. K) for 1.65 SG_M coal. Such observation infers that the combustion reactivity of 1.65 SG_M coal is superior to other coal fractions. Overall, thermodynamic analysis indicates that, for best-suited combustion characteristics with lower ΔH, ΔG and maximum ΔS, most functional groups and higher surface properties should be present in coal.

Figure 7

Variation in (a) ΔH, (b) ΔG and (c) ΔS with coal SG_M.

Conclusions

Different SG_M fractions of coal were generated using the float–sink experiments. Characterization results signified that, when SG_M of coal rose from 1.25 to 2.0, ash deteriorated from 2.41 to 76.36%, carbon reduced from 79.64 to 53.65% and HHV declined from 9606 to 6577 kcal/kg. Ash analysis inferred that heavier SG_M coals have higher slagging and fouling characteristics. FTIR provided insight into coal combustion characteristics and distribution of hydrocarbons with SG_M. The combustion rate of 1.25 SG_M coal was found higher due to stronger or moderate availability of most of the hydrocarbons such as alkane, alkene, aldehyde and alcoholic as well as aliphatic C–H stretching and aromatic nucleus of C=C as obtained from FTIR. Combustion experiments showed that, with progression of coal SG_M from 1.25 to 2.0, T_IN rose from 285 to 408 °C, T_PK ranged from 398 to 425 °C, and T_FL varied from 469 to 497 °C. The kinetic analysis signified that activation energy ranged from 121.81 to 54.60 kJ/mol as SG_M of coal varied from 1.25 to 2.0. Thermodynamic analysis inferred that 1.65 SG_M coal has a lower value of ΔH (48.93 kJ/mol) and ΔG (158.09 kJ/mol), inferring smooth decomposition during the coal combustion. Overall, the theoretical and experimental analysis suggested that 1.325 to 1.55 SG_M would be preferable for thermal power plants. This work thus observed variations in coal combustion characteristics with its mean specific gravity (SG_M).