3.1. Physicochemical properties
Table 1, presents the physicochemical properties of the AKZ, OME, and SHG coal samples. The elemental composition of carbonaceous materials such as coal is an indicator of its potential energy recovery, rank classification, and or evolved gas emissions during thermal conversion. As observed in Table 1, the ultimate analysis indicates that the coal samples contain the following range of elements: Carbon (C), 47.62–64.35 wt%; Hydrogen (H), 4.13–5.55 wt%; Nitrogen (N), 1.12–1.40 wt%; Sulphur (S), 0.57–0.69 wt%; and Oxygen (O), 28.01–46.56 wt%. The highest C and H were observed in SHG coal sample, while the lowest values were observed in AKZ. Furthermore, the N and S content indicate the conversion of the coals examined could potentially generate hazardous emissions of nitrous (NOx, NHx) and sulphurous (SOx, HxS where x = 1…n) gases during thermal conversion. In addition, the high carbon could potentially result in high emissions of greenhouse gases such as carbon dioxide (CO2) and carbon monoxide (CO). Consequently, the thermal conversion of these coals could potentially exacerbate climate change and global warming along with other greenhouse gas emissions, which pose severe risks to human health and safety. Hence, strategic measures that ensure high efficiency and low emissions are required for the design of future power plants based on the coals examined in this study (Akinyemi et al. 2019; Reddy 2013).
Table 1
Physicochemical Fuel Properties of the coals
Fuel Property | Symbol (Units) | Akunza Coal (AKZ) | Ome Coal (OME) | Shiga Coal (SHG) |
Carbon | C (wt.%) | 47.62 | 58.31 | 64.35 |
Hydrogen | H (wt.%) | 4.13 | 5.02 | 5.55 |
Nitrogen | N (wt.%) | 1.12 | 1.31 | 1.40 |
Sulphur | S (wt.%) | 0.57 | 0.66 | 0.69 |
Oxygen | O* (wt.%) | 46.56 | 34.69 | 28.01 |
Moisture | M (wt.%) | 6.00 | 6.35 | 6.23 |
Volatile Matter | VM (wt.%) | 26.18 | 32.18 | 37.74 |
Ash | A (wt.%) | 9.45 | 11.59 | 11.24 |
Fixed Carbon | FC* (wt.%) | 58.38 | 49.88 | 44.80 |
Higher Heating Value | HHV (MJ/kg) | 18.65 | 24.17 | 26.59 |
*Determined by difference |
Proximate analysis showed the coal samples contain the following range of properties: Moisture (M), 6.00–6.35 wt%; volatile matter (VM), 26.18–37.74 wt%; Ash (A), 9.45–11.59 wt%; and fixed carbon (FC), 44.80–58.38 wt%. The highest moisture content was observed in OME coal, whereas the lowest was in AKZ coal. The moisture content of coal is an important parameter that affects its ignitability and potential utilisation (Speight 2012). As such, high moisture is unsuitable for the processing and utilisation of coal. In this study, the moisture content of all the coals are below 10%, which is within the acceptable limits for electric power generation or conversion into value-added products (Akinyemi et al. 2020a; Chukwu et al. 2016). In contrast, the volatile matter (VM) is an important variable for determining not only the coal rank but also its suitability for potential energy recovery from thermal processes such as combustion and pyrolysis (carbonisation) (Nyakuma 2016; Speight 2012). In this study, the highest VM was observed for SHG coal, whereas the lowest was in AKZ. The findings indicate that SHG could be a more suitable feedstock for gasification compared to AKZ, which is more suited for pyrolysis into coke. The findings also showed that AKZ contains the highest fixed carbon (FC) content, which is typically inversely proportional to the VM content. The FC of coal is a measure of the approximate coke yield after devolatilization during carbonisation or slow pyrolysis. As such, it indicates that AKZ may be most suitable for pyrolysis, as earlier surmised, due to its low VM and high FC. Furthermore, the highest AC was observed in OME coal, whereas the lowest was observed in AKZ. Typically, ash represents the bulk mineral matter, inorganic or non-combustible residue arising from the coal combustion, which has potential impacts on the environment (Akinyemi et al. 2020b). According to various studies, the chemical composition of coal ash is crucial to coal conversion due to its influence on slagging, agglomeration or viscosity of bed materials (Cebeci et al. 2002; Özer et al. 2017). Based on the foregoing, it can reasonably be surmised that OME coal could potentially pose more ash related problems compared to SHG and AKZ during combustion.
The higher heating value (HHV) of the coals ranged from 18.65 MJ/kg to 26.59 MJ/kg, with the SHG sample reporting the highest value while the AKZ reported the lowest value. The HHV is a measure of the heat content or energy value of any coal sample (Speight 2012). It is also employed to predict the rank, classification, and or assess the suitability of any coal for various applications (ASTM D388-12 2012). Hence, it is regarded as one of the most important thermal or physical properties of coals. Based on the HHV, AKZ coal sample could be classified as Lignitic class or Lignite A coal with heating values typically in the range of 14.70 MJ/kg to 19.30 MJ/kg. The OME coal sample with its HHV of 24.17 MJ/kg could be ranked as Subbituminous or specifically classified as Subbituminous B coal which exhibits HHV from 22.10 MJ/kg to 24.40 MJ/kg. However, the SHG coal sample could be classified high-volatile C Bituminous agglomerating coal or Subbituminous A non-agglomerating coal, with HHVs typically ranging from 24.40 MJ/kg to 26.70 MJ/kg (Speight 2012). Based on the HHV, AKZ could be potentially utilised for electricity generation, whereas OME may be suitable for cement manufacture and SHG for the manufacture of metallurgical coke for iron and steel production. Overall, AKZ and OME are low-rank coals similar to Nigerian lignite coals from Obomkpa, Ihioma, and Ogboligbo (Nyakuma et al. 2019a; Nyakuma 2019) and Subbituminous coals such as Owukpa and Garin Maiganga (Nyakuma and Jauro 2016b; Ryemshak and Jauro 2013), whereas SHG is high-ranked coal similar to Shankodi-Jangwa (Nyakuma and Jauro 2016a).
3.2. Microstructure and Mineralogical Properties
The SEM micrographs and EDX spectra for the coal samples are shown in Figs. 3–5 (a & b). The results present insights into the surface morphology, microstructure, and chemistry of the constituent elements in the coal samples. For all the samples, the SEM micrographs displayed particles with rough textures and surfaces characterised by a distinct glassy sheen, which is typically ascribed to the presence of metallic elements, minerals, or aluminosilicates such as quartz and kaolinite (Karayigit et al. 2001). In addition, the scanned particles were composed of closely packed stratified layers of materials with contoured outlines owing to deposition of organic material during the coalification process. Lastly, the particles were found to be devoid of surface pores or crevices, which indicates a dense, compact and sintered microstructure.
The mineralogical properties of AKZ, OME, and SHG were examined by energy-dispersive X-ray (EDX) spectroscopy, as shown in Table 2. The elements detected in the coal samples were; Aluminium (Al), Carbon (C), Calcium (Ca), Copper (Cu), Iron (Fe), Potassium (K), Magnesium (Mg), Oxygen (O), Sulphur (S), Silicon (Si), and Titanium (Ti) at various concentrations. For all samples, the major elements detected were; C, O, Si, Al, whereas Ca, Cu, Fe, K, Mg, S, and Ti were detected in minor proportions. It is vital to state that Sulphur and Potassium were undetected in OME and SHG, respectively. Typically, the occurrence of metals is related to the clay, salt, or the porphyrin constituents in the coal structure, and they serve as a measure of the level of coalification (Speight 2012). For all cases, the major elements detected as defined by composition > 2.50 wt. %, were in the order C > O > Si > Al. The highest composition of C was detected in SHG, whereas the lowest was observed in OME. However, the highest and lowest compositions of O were observed in AKZ and SHG, respectively. The higher C but lower O of SHG compared to the other samples account for its high calorific value (26.59 MJ/kg) reported in Table 1. Hence, the results of the mineralogical study are consistent and agree with the physicochemical analyses.
Table 2
Elemental Compositions of AKZ, OME, SHG coal samples by EDX Analysis
Element | Symbol | Akunza (AKZ, wt.%) | Ome (OME, wt.%) | Shiga (SHG, wt.%) |
Aluminium | Al | 3.94 | 6.57 | 0.72 |
Carbon | C | 57.26 | 49.33 | 76.67 |
Calcium | Ca | 0.07 | 0.23 | 0.16 |
Copper | Cu | 0.09 | 0.24 | 0.24 |
Iron | Fe | 0.35 | 1.07 | 0.26 |
Potassium | K | 0.20 | 0.25 | 0.00 |
Magnesium | Mg | 0.06 | 0.07 | 0.05 |
Oxygen | O | 31.78 | 29.16 | 18.88 |
Sulphur | S | 0.05 | 0.00 | 0.18 |
Silicon | Si | 6.00 | 12.51 | 2.59 |
Titanium | Ti | 0.19 | 0.58 | 0.24 |
The highest composition of Si was observed in OME, whereas the lowest was observed in SHG. Typically, Si exists in coal in the form of silicon dioxide (SiO2) otherwise termed quartz (Speight 2012), which accounts for 40–90% of the major inorganic components of ash formed in coal and other combustible matter (Wong et al. 2020). Quartz is the primary constituent of various granite, quartz, porphyry, and rhyolite rocks and tends to occur due to proximity to coal beds during the process of silicate weathering or coalification (Akinyemi et al. 2020b; Speight 2012). Hence, the high Si indicates the presence of SiO2 in OME, which is in good agreement with the high ash content of OME as earlier reported in Table 1.
Similarly, the highest composition of Al was observed in OME, whereas SHG contains the lowest composition. Typically, Si and Al exist as clay minerals or aluminosilicates, which account for the highest inorganic constituents of coal (Gluskoter 1975). Furthermore, Al and Si also suggest the presence of carbonaceous and quartz minerals such as clay (Gluskoter 1967; Sellaro et al. 2015). The most common clay minerals are kaolinite, illite, chlorite, sericite, and montmorillonite (Liu and Peng 2015). In addition, the clay found in coals is a significant contributor to ash formation, loss of calorific value, and increased cost of ash handling/disposal during the combustion of coal in power plants (Spears 2000). Lastly, the presence of clay minerals along with other metal elements such as Ti and Fe (Sengupta et al. 2008) may account for the distinct lustre observed in the coals, as reported earlier in our previous study (Nyakuma et al. 2019b). In general, the minor elements detected were in the order Fe > K > Ti > Cu > Ca > Mg > S particularly for AKZ. For OME no sulphur (S) was detected and the composition of Ti > K, whereas for SHG no K was detected and the composition of S > Ca. The highest composition of Fe, Ti, K, Ca, and Mg was detected in OME, which indicates high mineral compositions of pyrite (FeS2), anastase or ilmenite (Ti), illite (K), calcite (CaCO3), and oxides of Mg.
3.3. Thermal Degradation Properties
The thermal properties of AKZ, OME, and SHG were examined under oxidative and non-oxidative conditions based on non-isothermal heating to examine the burning (combustion, CMB) and devolatilization (pyrolysis, PYR) profiles of the coal samples, as depicted in Figs. 6 and 7.
The burning and devolatilization profiles of the coals depicted in the TG plots showed the typical downward “Z” curves, which slope from left to right for most thermally degrading carbonaceous materials. The findings indicate that the non-isothermal increase in temperatures from 30 °C to 900 °C resulted in significant thermal degradation during TGA. As observed in Fig. 6, the burning profiles resulted in steeper plots particularly between 300 °C and 500 °C compared to the devolatilization profiles (Fig. 7). The steeper TG plots (Fig. 6) observed for the burning profiles indicate more significant thermal degradation, loss of mass, and mass-loss rates in the coals compared to during the devolatilization process. This is ascribed to the exothermic nature of the oxidative (combustion) process, which ensures the higher heat or thermal energy supply to the coal particles during the TG degradation process.
The thermal degradation observed during TGA could also be ascribed to the degradation of the organic fractions or maceral components of coals. The term macerals describe the microscopic, and rock-rich constituents of coal comprising the vitrinite, inertinite, and liptinite groups. Typically, the compositions range from 50–90% for vitrinites, 5–10% for liptinites, and 50–70% for the inertinites depending on the rank, classification, and source of the coal. Furthermore, the macerals are physico-chemically and structurally comprised of polymers, lignin, cellulose, resins, spores, and cuticles derived from plants, algae, and fungi residues (Speight 2012; Sun et al. 2003; Xie et al. 2013). Hence, the loss of mass during TGA could be ascribed to the thermal degradation of plant cell wall matter (or organic fractions) present in the coal samples. Košina and Heppner (1984); Landais et al. (1989) demonstrated that the degree of the thermal degradation, physicochemical behaviour, and potential conversion products greatly depends on the maceral composition, rank, and atomic ratios of coals. Hence, the effect of the oxidising and non-oxidising environments on the thermal degradation of AKZ, OME, and SHG was examined by temperature profile characterization. Table 3 presents the temperature profile characteristics (TPCs) of the coals under oxidative (combustion) and non-oxidative (pyrolysis) conditions during TGA.
Table 3
TG plot - Temperature Profiles Characteristics of the Coal Samples
Coal Sample | Reaction/ Process | Temperatures (°C) | Mass Loss (ML, %) | Residual Mass (RM, %) |
Onset (Tons, °C) | Midpoint (Tmid, °C) | Endset (Tend, °C) |
AKZ | Pyrolysis | 363.35 | 480.98 | 594.32 | 30.51 | 69.49 |
Combustion | 349.64 | 395.95 | 436.66 | 69.61 | 30.40 |
OME | Pyrolysis | 385.95 | 468.24 | 556.11 | 39.04 | 60.96 |
Combustion | 361.05 | 405.59 | 440.24 | 81.71 | 18.29 |
SHG | Pyrolysis | 391.13 | 468.55 | 543.03 | 43.05 | 56.95 |
Combustion | 374.93 | 421.12 | 459.39 | 87.57 | 12.44 |
As observed, the oxidative conditions resulted in a high loss of mass ranging from 69.61% for AKZ to 87.57% as observed for SHG. Due to the oxidative nature of the process, it can be reasonably inferred that the mass loss during the process results in flue gas mixture along with coke and ash, which are collectively termed the residual mass. In this study, the residual masses for the oxidative process ranged from 12.44–30.40% as observed for SHG and AKZ, respectively. Compared to the ash contents of the coals in Table 1, it can be reasonably surmised that the residual mass comprises largely ash (9.45 wt.% to 11.59 wt.%) along with the coke or unreacted coal particles arising from incomplete combustion during TGA. Hence, higher temperatures (above 900 °C), longer residence and isothermal conditions are required for complete combustion of the coals.
For the non-oxidative conditions, the mass loss ranged from 30.51% for AKZ to 43.05% as observed for SHG, which resulted in the residual mass ranging from 56.95% for SHG to 69.49% for AKZ. Based on the nature of the process, the predicted products of the mass loss could be pyrolysis gas (fuel gases), oil, and tar, whereas the solid products could be largely coke, char and ash. This view is corroborated by Sun et al. (2003) who examined the pyrolysis behaviour of Shenmue coals. The findings showed that coal pyrolysis resulted in light hydrocarbons (C1-C4), aromatic hydrocarbons (C6-C8) along with carbon dioxide (CO2), water vapour (H2O) and other pyrolysis gases due to the thermal degradation of macerals such as vitrinite and inertinite. According to the authors, the yield and composition of the pyrolysis product gas are significantly dependent on the reactivity of the macerals (Sun et al. 2003). Similarly, Zhao et al. (2011), employed TG–MS and a fixed bed reactor to examine the pyrolytic decomposition behaviour of Pingshuo coal. The findings indicated that pyrolysis of coals is largely dependent on the reactivity of individual macerals particularly inertinite compared to vitrinite. Furthermore, the study showed that the distribution of pyrolysis products is comprised of tar, fuels gas, and char. These findings are corroborated by Zou et al. (2017) whose study showed that coal pyrolysis results in a fuel gas mixture comprising; hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) and water vapour (H2O) and ethylene (C2H2) based on the TG-MS and gas evolution. Overall, the mass loss for the oxidative process in this study was higher than the non-oxidative process, whereas the residual masses were higher during the non-oxidative compared to the oxidative processes. The plausible explanation can be found in the higher thermal energy and exothermic nature of the oxidative process, which provides the heat required to break the bonds of the macerals in the coal structure. This assertion is verified by the higher mass-loss rates observed during the oxidative thermal degradation (17.70%/min to 19.91%/min) compared to the non-oxidative process (2.26%/min to 5.3%/min) along with other TPC values shown in Table 3.
As observed in Table 3, the onset or ignition (Tons) temperatures for the oxidative coal degradation processes commenced from 349.64 °C (AKZ) to 374.93 °C (SHG), whereas the values for the non-oxidative process occurred between 363.35 °C (AKZ) and 391.13 °C (SHG). However, the endset or burnout (Tend) temperatures for the oxidative coal degradation processes occurred from 436.66 °C (AKZ) to 459.39 °C (SHG), whereas the values for the non-oxidative process occurred between 543.03 °C (SHG) and 594.32 °C (AKZ). For all cases, the TPC values Tons, Tmid and Tend were higher for the non-oxidative degradation of the coals compared to the oxidative process. As earlier surmised, this is due to the exothermic nature of the oxidative process, which provides higher heating energy and hence higher mass-loss rates required to thermally degrade the coal components compared to the non-oxidative process during TGA. Furthermore, the oxidative reaction conditions provide suitable conditions for the secondary cracking or thermal degradation of condensable products and char/coke produced during the TGA process.
The degradation pathway for the oxidative and non-oxidative thermal degradation of the coals was examined by derivative thermogravimetry (DTG) plots, as shown in Figs. 8 and 9.
The DTG plots in Figs. 8 and 9 each reveal two sets of symmetric and asymmetric peaks. The first sets of smaller peaks occurred from 30 °C to 200 °C for the oxidative and non-oxidative processes, whereas the second larger set of peaks were from 200 °C to 500 °C for the oxidative and 200 °C to 600 °C for the non-oxidative thermal degradation. The non-oxidative process occurred over a wider temperature range compared to the oxidative process. This is verified by the temperature difference of the Tons and Tend for the non-oxidative thermal degradation, which was; 230.97 °C, 170.16 °C, and 151.90 °C for AKZ, OME, and SHG, respectively, compared to 87.02 °C, 79.19 °C, and 84.46 °C for the oxidative process. The peaks for the oxidative process were found to be asymmetric with shoulder protuberances on the left-hand side of the large peak between 300 °C and 350 °C for all samples and between 400 °C and 450 °C for the OME and SHG coal samples. In contrast, the non-oxidative process resulted in symmetric peaks devoid of shoulder peaks between 200 °C and 600 °C. Furthermore, the observed peaks for the non-oxidative process exhibited lower derivative mass-loss rates (Table 4) and significantly smaller peaks from 200 °C to 600 °C compared to the oxidative process. This observation indicates the devolatilization process, which governs thermal degradation and softening is largely endothermic, as similarly reported in the literature (Agroskin et al. 1972; Hanrot et al. 1994).
Based on the thermal degradation ranges, peak sizes and symmetry, it can be reasonably inferred that the oxidative and non-oxidative thermal degradation processes occur in three (3) stages. The first stage could be ascribed to drying or loss of coal surface moisture along with low molecular weight volatile components below 200 °C (Xie et al. 2013). Zou et al. (2017) reported that the loss of mass during this stage of coal degradation is also ascribed to the evolution of moisture, free radical groups, and hydrogen (H2). Accordingly, the second stage observed between 200 °C and 500 °C and from 200 °C to 600 °C for the oxidative and non-oxidative processes, respectively, could be attributed to the bond cleavage or cracking of tar along with the evaporation and transport of evolved gases during the thermal degradation of coal macromolecules (Zou et al. 2017). Likewise, this stage could also be due to the thermal degradation of coal components such as macerals, as earlier surmised.
Hence, the mass loss during the TGA of the coals in this study could be largely due to vitrinite, which is the most abundant maceral fraction compared to inertinite and liptinite in decreasing order. In addition, the high temperature (typically 400–800 °C) degradation of coal is considered an exothermic process which is mainly due to vitrinite degradation alongside coke graphitization (Xie et al. 2013) and contraction due to dehydrogenation of organic matter (Landais et al. 1989). Consequently, the thermal conductivity of the coal increases resulting in enhanced fluidity, swelling, and meta-plasticity (Strezov et al. 2007; Xie et al. 2013). However, the mass loss during thermal degradation in the range 200 °C < x < 400 °C has been previously ascribed to the degradation of inertinite macerals, which occurs at low temperatures in the coal structure (Xie et al. 2013). Landais et al. (1989) reported the low reactivity of inertinite to degradation in the range of 350 °C and 375 °C under pyrolysis conditions. Typically, the weight loss path and mechanism during high-temperature coal degradation proceeds from inertinite to vitrinite and lastly liptinite (Landais et al. 1989). The last stage (> 500 °C and 600 °C) resulted in low mass losses for both oxidative and non-oxidative processes which is evident in the tailing (mass loss plateaux) observed in Figs. 8 and 9. Various researchers have ascribed the mass loss in this region to the decomposition of mineral matter and condensation of aromatic rings due to secondary degassing reactions, which occur at high temperatures (Zou et al. 2017).
The temperature profiles characteristics (TPC) for the DTG plots were deduced to examine the reactivity, mechanism and mass-loss rates for the drying and devolatilization processes under oxidative and non-oxidative conditions, as presented in Table 4.
Sample
|
Reaction
|
Peak Drying
Temp (°C)
|
Drying Rate
(%/min)
|
Peak Devolatilization
Temp (°C)
|
Devolatilization
Rate (%/min)
|
AKZ
|
Pyrolysis
|
77.27
|
1.26
|
475.95
|
2.26
|
Combustion
|
65.87
|
1.71
|
385.44
|
17.7
|
OME
|
Pyrolysis
|
63.97
|
1.82
|
470.29
|
4.02
|
Combustion
|
67.83
|
1.62
|
389.80
|
19.91
|
SHG
|
Pyrolysis
|
60.38
|
1.79
|
470.67
|
5.13
|
Combustion
|
68.27
|
1.62
|
405.26
|
19.3
|
Table 4
DTG plot - Temperature Profiles Characteristics
For all cases, the peak drying temperatures were observed from 60.38 °C (SHG) to 77.27 °C (AKZ) for the non-oxidative process compared to the oxidative process, which was observed from 65.87 °C (AKZ) to 68.27 °C (SHG). However, mass loss rates for both oxidative and non-oxidative conditions were similar indicating the reactivity of the coals was similar during the drying stage. However, the peak devolatilization temperatures were observed from 470.29 °C (OME) to 475.95 °C (AKZ) for the non-oxidative process compared to the oxidative process, which was observed from 385.44 °C (AKZ) to 405.26 °C (SHG). However, the mass-loss rates for both oxidative and non-oxidative conditions were markedly different indicating the mechanism and thermal reactivity of the coals. The findings indicate that thermal degradation behaviour of coals is also significantly dependent on the nature of the oxidising environment as similarly observed for the composition of the organic materials or maceral components in the literature.