Energy Recovery Potential of Obomkpa Coal

This study presents preliminary findings on the physicochemical, calorific, and thermal properties of a newly discovered coal from Obomkpa town in Aniocha-North Local Government Area of Delta State in Nigeria. The Obomkpa (BMK) coal sample was subjected to fuel characterisation through ASTM standards and techniques for determining elemental, proximate, and calorific coal properties. The results indicated that BMK coal contains high carbon, oxygen, volatile matter, fixed carbon, and ash. The higher heating value of BMK was 19.66 MJ/kg, which indicates it could be ranked as either lignite A or subbituminous C coal. The thermal properties indicated that BMK experienced significant mass loss (ML) under oxidative (combustion) and nonoxidative (pyrolysis) conditions due to thermal degradation of organic rock macerals (inertinite and vitrinite). Furthermore, BMK experienced mass loss (ML=59.27%–76.56%) along with residual mass (RM = 23.44–40.73%) under oxidative (combustion) and non-oxidative (pyrolysis) conditions. Thermal degradation occurred in three (3) stages; drying (30 °C to 200 °C), Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 April 2020 doi:10.20944/preprints202004.0344.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license. 2 devolatilization (200 °C – 500 °C and 600 °C), and lastly, coke degradation and ash formation. Furthermore, the DTG peaks for the drying and devolatilization of BMK during the oxidative (combustion) process occurred at lower temperatures compared to the non-oxidative (pyrolysis) process. Hence, the results submit oxidative (combustion) is a more thermally efficient process compared to the non-oxidative (pyrolysis) process. Lastly, the findings indicate BMK is a potential feedstock material for future coal power generation, steel, iron ore or cement production.


Introduction
Coal is an organic, combustible, and sedimentary rock formed from several years of metamorphic reactions on the plant and animal remains in the earth's crust (Speight, 2013). As a result, coal is considered an important fossil fuel with high carbon, fixed carbon, and higher heating values with various potential applications (Miller, 2016). Over the years, coal has become an integral part of the global energy mix due to its low processing costs, widespread availability, and geographically distribution on the planet (IEA, 2018). Currently, coal accounts for 38-41% of all electricity generated globally (BP, 2018). Coal-fired electricity is typically generated through pulverised coal combustion (PCC), which utilises various ranks of coal (Nimmo et al., 2010;Al-Abbas et al., 2013;Dai et al., 2015).
Over the years, numerous deposits of coal have been discovered around the globe particularly in developing countries. Most notably, there are numerous reports of new coal deposits in Nigeria Nyakuma, 2016;Nyakuma et al., 2018a), which is the largest economy and most populous nation in Africa. The utilization of coal for power generation presents 3 significant opportunities for rapid socio-economic growth, infrastructural development, and poverty alleviation (Musa, 2010;Oseni, 2011;Oboirien et al., 2018). Currently, Nigeria experiences perennial power cuts and load shedding due to numerous challenges such as low generation along with inefficient distribution, and transmission (Odior and Oyawale, 2012;Odior and Omadudu, 2013). Hence, the underlying problems of power generation highlighted in the literature (Oyedepo, 2014;Ocheri, 2017;Denloye and Akinola, 2017) could be potentially addressed through coal utilization for power generation in Nigeria.
Conversely, the utilization of coal for electric power generation requires comprehensive data on the fuel properties, emissions profile, life cycle, environmental impact, and engineering economics among others. Typically, the physicochemical, calorific, and thermal properties of coals avail designers, engineers, policy/decision-makers with valuable data required for the planning, design, and development of coal-fired power plants. In addition, the outlined properties of coal provide insights into the thermal degradation behaviour, emissions profile, and potential environmental impact during energy recovery and electricity generation in power plants. However, the data on various Nigeria coals is limited to the geological, geochemical, petrographic, petrologic, sedimentology, and mineralogical properties as reported in the literature (Ayinla et al., 2017;Ogala, 2018;Akinyemi et al., 2020).
Therefore, this study seeks to present preliminary data on the fuel properties of a newly discovered coal from Obomkpa (BMK) town in Aniocha-North Local Government Area of Delta State in Nigeria. Consequently, the physicochemical and calorific properties of BMK coal were characterised through selected ASTM standards, techniques and equipment. The thermal properties were examined under a non-isothermal program to simulate the flash (high heating rates) conditions typically observed in large scale coal conversion in coal-fired power plants.

Experimental
This section presents the materials and methods employed to examine the fuel and energetic properties of BMK coal. The coal sample was obtained from Obomkpa town in Aniocha-North Local Government Area of Delta State in Nigeria. The sampling and sample preparation methods are presented in the previous study (Nyakuma et al., 2018c).

Physicochemical Analysis
The physicochemical analysis of BMK was examined through ultimate, proximate, and calorific analyses. Firstly, the pulverised BMK of particle size < 250 µm was examined through an elemental analyser (vario MACRO Cube, Germany) to determine its carbon (C), hydrogen (H), nitrogen (N), and sulphur (S) compositions based on the ASTM standard D5373. Subsequently, the proximate analysis was determined by thermogravimetric analysis (TGA) based on the procedure of Donahue and Rais (2009). The moisture (MC) and volatile matter (VM) contents were determined at 110 °C and 900 °C, respectively under nitrogen (N2) atmosphere, whereas the ash content (AC) was determined in air atmosphere at 900 °C. The fixed carbon (FC) content was subsequently determined by difference from the sum of MC, VM, and AC previously determined by TGA. Lastly, the calorific value was determined by bomb calorimetry through the isoperibol bomb calorimeter (IKA C200, USA). Each test was performed in duplicate to ensure the reliability and accuracy of the averaged measurements presented in Table 1.

Thermal Analysis
The thermal analysis and energy recovery potential of BMK were investigated by thermogravimetric (TG) analysis. For each test run, 17-20 mg of BMK was weighed in alumina 5 crucible before transferring to the TG analyser (Shimadzu TG-50, Japan) for thermal analysis.
Next, the sample was heated under the selected non-isothermal heating program from room temperature or 25 °C to 900 °C based on the flash heating rate of 50 °C/min. For the flash oxidative (combustion) conditions, the sample was heated under air atmosphere (flow rate = 20 mL/min), whereas the flash non-oxidative (pyrolysis) process was performed under nitrogen (N2) (flow rate = 20 mL/min). After the processes were completed, the raw thermogram (.tad) data were analysed on the thermal analysis software (Shimadzu Workstation TA-60WS, Japan). The resulting mass loss and derivative mass loss data were retrieved and plotted against temperature to obtain the TG, DTG, conversion plots presented in Figures 1-3.

Temperature Profile Analysis
The thermal degradation of BMK was further examined through temperature profile analysis. Hence, the temperature profile characteristics (TPC) were deduced for the thermal degradation of BMK under oxidative (combustion) and non-oxidative (pyrolysis) TG conditions.
For this study, the TPCs deduced from the TA-60WS software were; the onset or ignition temperature (Tons), midpoint temperature (Tmid), burnout or offset temperature (Toff), mass loss (ML, %) and residual mass (RM, %) from the TG plots in Figure 1. However, the drying and devolatilization peak temperatures were deduced from the DTG plots in Figure 3. Based on the

Results and Discussion
The fuel properties and energy recovery potential of BMK coal are presented based on its ultimate, proximate, calorific, and thermal properties.  (Vassilev et al., 2015;Parvez et al., 2017;Nyakuma et al., 2018b).  (Speight, 2012). Similarly, the study by Ogala et al. (2012) examined the petrographic, mineralogical and geochemical properties of lignites from Ogwashi-Asaba in the Delta state of Nigeria. The findings showed that the compositions of C, H, N, and S are comparatively higher than BMK examined in this study. However, the somewhat marked differences observed could be to the level of coalification, origins, sampling, treatment or preparation methods of the coals, despite their similar rank and classifications.

Physicochemical Properties
Conversely, the proximate properties VM and FC of BMK were found to be in fairly good agreement with Ogala et al. (2012), although the MC and AC were markedly dissimilar. The AC was found to be in good agreement with raw lignite sample examined by Agraniotis et al. (2012).
Generally, the observations of this BMK study and the findings in the literature indicate that the mode of sampling, pre-treatment and preparation plays a crucial role in the proximate fuel properties. Lastly, the calorific value of BMK was found to be in fairly good agreement with other lignite coals reported in the literature (Agraniotis et al., 2012;Nyakuma, 2019a;Nyakuma, 2019b).

Thermal Properties
The thermochemical analysis and energy recovery potential of BMK was investigated by thermogravimetric analysis (TGA). The resulting TG, conversion, and DTG plots for the oxidative (combustion) and non-oxidative (pyrolysis) processes are shown in Figures 1-3.   The TG plots indicate BMK coal experienced significant mass loss as evident in the downward sloping plots which can be seen from left to right of Figure 1. The findings indicate the increase in temperature resulted in significant thermal degradation of BMK. Consequently, the rate of conversion ( Figure 2) increased with increasing temperature during the process irrespective of the oxidising environment in which the process occurred. Typically, coal degradation is ascribed to the breakdown of the organic matter or macerals (namely; inertinite, vitrinite and liptinite) in the coal structure (Zou et al., 2017). During the process, the inertinite degrades at temperatures below 400 °C, whereas the vitrinite fraction which accounts for the largest thermally degraded component occurs between 450 °C and 600 °C (Landais et al., 1989;Xie et al., 2013).
The thermal degradation pathway for BMK was also examined through the DTG plots presented in Figure 3. As observed, the thermal degradation of BMK resulted in various peaks indicating various reactions or processes occurred under both oxidative and non-oxidative TG conditions. The first set of peaks can be observed in the temperature range from 30 °C to 200 °C corresponding to mass losses of 4.72% and 4.76% for the oxidative (combustion) and non-oxidative (pyrolysis) processes, respectively. This mass loss observed during the thermal degradation processes is in good agreement with the moisture content (3.80%), indicating it could be ascribed to drying or evaporation of surface moisture on BMK.
However, the second set of peaks were observed from 200 °C to 475 °C for the oxidative (combustion), which consists of two peaks with maximum values observed at 326.58 °C and 425 °C. For the non-oxidative (pyrolysis) process, the single peak occurred between 200 °C and 550 °C. In comparison, the peaks in the second stage for the oxidative (combustion) process were observably smaller in size and somewhat symmetric compared to the non-oxidative process, which confirms the exothermic and endothermic nature of the degradation processes, respectively.

Temperature Profile Characteristics
The effects of the oxidative (combustion) and non-oxidative (pyrolysis) conditions on the thermal degradation of BMK were examined by temperature profile characteristic (TPC) analysis.
Tables 2 and 3 present the TPCs of BMK deduced from the TG and DTG plots, respectively. As observed, the BMK degradation process under oxidative (combustion) conditions resulted in higher mass loss of 76.56% compared to 59.27% for the non-oxidative process. This could be ascribed to the exothermic nature of the oxidative process, as earlier reported, which provides higher thermal energy required to degrade the coal maceral components compared to the nonoxidative process. Furthermore, the high mass loss could be ascribed to the higher temperature differences observed between the onset and burnout temperatures of both processes.  As a result, the residual mass for the non-oxidative process was higher than the oxidative process. Typically, the oxidising environment and the residual mass of the processes is an indicator of the nature of the final products after thermal degradation. For oxidative (combustion) processes, the resulting mass of residuals is largely due to ash residues (Vassilev et al., 2013;Nyakuma et al., 2019). However, since the ash content for BMK is 16.38% (Table 1), it can be reasonably surmised that the residual mass from the oxidative TG process could also be due in part to coke (7.06%) along with ash and mineral matter. On the other hand, the residual mass of the nonoxidative process may be largely due to coke (24.35%) along with ash/ mineral matter. However, extensive coke or carbonization tests are required to empirically confirm the outlined submissions.
The TPCs for the DTG plots were also examined as presented in Table 3. As observed, the thermal degradation processes could be described in terms of drying, devolatilization, and coke degradation/ash formation depending on the oxidising nature of the process. The results indicate that the peaks for the drying and devolatilization of BMK during the oxidative (combustion) process occurred at lower temperatures compared to the non-oxidative (pyrolysis) process. As earlier surmised, these observations could be ascribed to the higher energy input and exothermic nature of the combustion process, which ensured a higher rate of thermal degradation of BMK coal components. This view is also corroborated by the higher peak drying (2.91%/min) and devolatilization (7.60 %/min) rates observed during the oxidative (combustion) process compared to the non-oxidative process.
Overall, the findings indicate that oxidative (combustion) process is more thermally efficient and could be utilised for future energy recovery or thermal energy applications. However, the nonoxidative (pyrolysis) process could also be beneficial for coke or carbon materials production for various applications such as steel, iron ore or cement production. However, further tests are required to examine the properties of BMK for the proposed applications outlined in these preliminary studies.

Conclusion
The study presented preliminary findings on the fuel properties and energy recovery potential of Obomkpa (BMK) coal from Delta State in Nigeria. Consequently, the study examined the physicochemical, calorific, and thermal properties of BMK. The ultimate, proximate, and process. However, further tests are required to examine the fuel properties of BMK for various potential applications such as electric power and production of materials.