Thermal Oxidative and Non-Oxidative Degradation Behaviour of Afuze Coal

This study presents a preliminary analysis of the chemical and thermal fuel properties of Afuze (AFZ) coal extracted from coalfields in Owan East Local Government Area of Edo State, Nigeria. The chemical properties of AFZ were examined by combined scanning electron microscopy-energy dispersive X-ray (EDX), whereas the thermal properties were deduced by thermogravimetric analysis (TGA) under flash (50 °C/min heating rate) oxidative (combustion) and non-oxidative (pyrolysis) conditions. The microstructure and morphology analysis of AFZ revealed has a compact structure comprising small-to-large, irregular shaped and exfoliated grains with a vitreous appearance typically ascribed to metal elements (Ti and Fe) kaolinite, quartz, and other clay minerals. Chemical analysis revealed carbon, oxygen, aluminium, silicon, sulphur, calcium, titanium, and iron in major and minor (trace) quantities. Thermal analysis under oxidative and non-oxidative conditions revealed degradation occurs in three stages, namely; drying or demineralisation, devolatilization or maceral degradation and the formation of char/coke or ash. Lastly, the findings showed that the temperature range for the oxidative thermal degradation process (338.58 756.76 °C) was higher than the non-oxidative process (378.43 615.34 °C). This observation can be explained by the exothermic nature of the oxidative (combustion) process, which ensures greater heat supply required to thermally soften or degrade the maceral coal components. Overall, the oxidative process yielded the residual mass (RM = 21.97%) and mass loss (ML = 78.03%). The lower ML (49.03%) but higher RM (50.97%) observed during nonoxidative degradation of AFZ could be ascribed to the largely endothermic nature of the process.


Introduction
Coal is a carbon-rich, brown to black, and energy-dense sedimentary rock that is widely utilised for electricity, cement, and steel production worldwide (Reddy, 2013;Speight, 2012).
Current estimates indicate that the consumption of coal accounts for over 60% of all global fossil fuels, which makes it the largest source of primary fossil fuel utilized for electric power generation (Sarwar et al., 2014;Nyakuma, 2015). Despite its high industrial value, there have been growing calls for the phase-out of coal due to its negative impacts on the environment (Heinrichs et al., economics, waste profile and environmental analyses of future coal-fired power plants in the country. Therefore, this study presents a preliminary analysis of the chemical and thermal fuel properties of Afuze (AFZ) coal. The research works on the AFZ coal sample, which was extracted from coalfields in Afuze located in Owan East Local Government Area of Edo State, Nigeria, are limited in the literature.

I. Chemical Analysis
The chemical composition of Afuze (AFZ) coal was examined by combined scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy. For each test, the pulverised AFZ was evenly spray-coated on carbon epoxy tape placed on sample grain mounts to disperse the coal particles. Next, the samples and grain mounts were transferred to the sample chamber of the SEM (Model: JEOL, JSM IT-300, Germany) for analysis. Next, the sample was scanned using the SEM microscope and a selected region of the resulting micrograph was mapped using the point ID technique. Next, the metallic and non-metallic elements in AFZ coal were computed in weight per cent (wt. %) based on charge balance using the EDX software (AZTEC Oxford Instruments, UK). A detailed description of the SEM/EDX analyser and its operating conditions are presented in the literature (Nyakuma et al., 2021).

II. Thermal Analysis
The thermal properties and degradation behaviour of AFZ coal were determined by thermogravimetric analysis (TGA). For each test, approximately 15 mg of pulverised AFZ was weighed in an alumina crucible and transferred to a sample bucket of the TG analyser. To examine the thermal-oxidative and non-oxidative degradation behaviour of AFZ, the sample was heated under air and nitrogen gas, respectively, from room temperature (RT) to 900 °C under a nonisothermal heating program at 50 °C/min heating rate at the flow rates of 100 mL/min. The objective was to examine the thermal behaviour and degradation mechanism of AFZ under flash combustion (oxidative in air) and pyrolysis (non-oxidative in nitrogen) conditions. In the end, the raw data was processed to determine the mass loss (ML, %) and derivative of mass loss (DML, %/min), which were subsequently plotted against temperature (°C) in Microsoft Excel (version 4 2013) to obtain the thermogravimetric (TG) and derivative thermogravimetric (DTG) plots presented in Figures 2 and 3 respectively.

III. Temperature Profile Analysis
Based on the TG-DTG plots in Figures 2 and 3, the characteristic temperature profiles of the AFZ under oxidative and non-oxidative conditions were determined using the Shimadzu software (Workstation TA-60WS, Japan). This feature of the software allows users to examine the thermal properties of thermally degrading materials through analysis of the characteristic temperature profiles after TGA. In this study, the temperature profile characteristics (TPCs) derived from the TG plots for AFZ degradation were; ignition temperature (Ti), midpoint temperature (Tm), offset temperature (Tf), mass loss (ML, %) and residual mass (RM, %), whereas the maximum drying peak (Td) and peak decomposition temperature (Tx) were deduced from the DTG plots. A detailed description of the procedures and guidelines for determining the TPCs are outlined in the literature (Nyakuma et al., 2021).

I. Chemical Analysis
The composition of metallic and non-metallic elements in AFZ coal was determined by energy-dispersive X-ray (EDX) spectroscopy computed in weight per cent (  Next, the composition of metal and non-metallic elements present in the AFZ coal structure were then determined by charge balance as shown in Table 1. As observed, the chemical structure of AFZ is characterised by the elements; carbon, oxygen, aluminium, silicon, sulphur, calcium, titanium, and iron. Based on the results, the major elements (defined as composition > 5 wt. %) in the AFZ coal structure are carbon and oxygen, which highlight the organic origins of the sample.  (SiO2), which is considered one of the most abundant minerals in the earth's crust (Bi et al., 2021;Kim et al., 2018). The occurrence of sulphur and iron may be due to FeS2 (iron sulphide), whereas calcium may be due to CaSiO3 (calcium ino-silicate mineral or Wollastonite). Overall, the presence of metallic and non-metallic elements are crucial to the assessment of the origins (e.g. petrographic source rock characteristics and depositional settings) (Ayinla et al., 2017a;Ayinla et al., 2017b;Ogala, 2011;Ogala, 2012;Ogala et al., 2012), properties (e.g. physicochemical, geochemical, thermal degradation) (Nyakuma et al., 2018;Nyakuma and Jauro, 2016a;Ogala, 2018), and potential applications (e.g. hydrocarbon generation potential) of coal (Akinyemi et al., 2021;Akinyemi et al., 2020b).

Figures 2 and 3 present the TG and DTG plots for the thermal-oxidative (combustion) and
non-oxidative (pyrolysis) degradation AFZ under flash conditions. The plots showed that the increase in temperature from RT to 900 °C resulting in significant mass loss during TGA. The loss of mass of AFZ could be ascribed to the thermal degradation of the maceral components (namely; vitrinite, inertinite, and liptinite) of the coal sample. The composition of vitrinites, liptinites, and inertinites typically range from 50% to 90%, 5%-10%, and 50%-70% based on source, rank, and classification of the coals (Speight, 2012;Nyakuma et al., 2021). Therefore, the mass loss of AFZ could be attributed to the thermal degradation of the maceral, lignitic and cellulosic plant materials (or organic fractions) or components present in the coal structure (Hayatsu et al., 1986;Košina and Heppner, 1984;Landais et al., 1989;Wu et al., 2017).
Similarly, the DTG plots showed that temperature affected the thermal degradation of AFZ during TGA. As observed, the process resulted in various endothermic peaks that occurred in the temperature ranges from RT -200 °C, 200 °C -600 °C, and 600 °C -900 °C. The findings indicate the process of AFZ degradation occurred in three (3) stages, which is similar to the findings reported in the literature for various ranks of coal Akinyemi et al., 2020b;Nyakuma and Jauro, 2016b). The mass loss in stage I (RT -200 °C) could be ascribed to the loss of surface-bound moisture and mineral hydrates in the coal structure (Sarwar et al., 2014), whereas stage II (200 °C -600 °C) could be due to the degradation of the coal macerals (Sun et al., 2003;Zhao et al., 2011). According to Sun et al. (2003) and Zhao et al. (2011), the reactivity of the macerals particularly inertinite and vitrinite play crucial roles in determining the yield and composition of the thermal degradation products. Lastly, stage III (600 °C -900 °C) may be due to the formation of char/coke or ash during the non-oxidative or oxidative degradation of AFZ, respectively.

III. Temperature Profile Analysis
The thermal degradation behaviour and reactivity of AFZ were further examined by analysing the temperature profile characteristics (TPCs) for the thermal-oxidative (combustion) and non-oxidative (pyrolysis) degradation of AFZ under flash conditions. In this study, the TPCs determined from the TG plots for AFZ degradation were; ignition temperature (Ti), midpoint temperature (Tm), offset temperature (Tf), mass loss (ML, %) and residual mass (RM, %). Table   2 shows the TPCs determined from the TG plots in Figure 2. findings indicate that the oxidative process exhibited a lower Ti but higher Tf when compared to the non-oxidative process. Intrinsically, the range of the degradation temperature range for the oxidative process was higher, which resulted in a higher mass loss (ML = 78.03%) and lower residual mass (RM = 21.97%) compared to the non-oxidative process. This could be explained by the exothermic nature of the oxidative (combustion) process, which has adequately greater heat required to thermally soften or degrade the chemical bonds of the maceral components in the coal structure. Likewise, the lower ML but higher RM observed during non-oxidative degradation of AFZ could be ascribed to the largely endothermic nature of the process, as reported in the literature (Agroskin et al., 1972;Hanrot et al., 1994). Similar findings were reported for various ranks of coals from Nigeria (Sonibare et al., 2005;Chukwu et al., 2016;Nyakuma et al., 2021). Table 3 shows the maximum drying peak (Td) and peak decomposition temperature (Tx) deduced from the DTG plots in Figure 3. Except for DPR, all the TPCs (Td, Tx, and PDR) for the non-oxidative (pyrolysis) are greater than observed for the oxidative (combustion) process.

Conclusion
The study examined the chemical and thermal properties of Afuze coal. The chemical analysis revealed that AFZ contains major and minor (trace) elements comprised of carbon, oxygen, aluminium, silicon, sulphur, calcium, titanium, and iron. The major elements (defined as composition > 5 wt. %) are carbon and oxygen, whereas the minor elements (defined as composition < 5 wt. %) detected were; silicon, sulphur, aluminium, iron, titanium, and calcium.
The thermal analysis of AFZ under oxidative and non-oxidative conditions under flash (50 °C/min heating rate) conditions provided insights into the thermal behaviour and degradation mechanism of AFZ. Furthermore, the findings revealed that the thermal degradation of AFZ occurred in three stages due to drying (loss of moisture) or demineralisation, devolatilisation or maceral degradation and the formation of char/coke or ash during the processes. Lastly, the findings showed that the degradation temperature range for the oxidative process was higher when compared to the nonoxidative process. Overall, the oxidative process yielded lower residual mass (RM = 21.97%) but higher mass loss (ML = 78.03%).

Acknowledgement
The author acknowledges Dr TAT Abdullah of the Centre of Hydrogen Energy and Institute of Future Energy at the Faculty of Engineering of Universiti Teknologi Malaysia (UTM) for the assistance with the TGA tests. The contribution of the University-Industry Laboratory of UTM is also gratefully acknowledged.