Combustion Kinetics of Shankodi-Jangwa Coal

The lack of comprehensive data on the fuel properties of newly discovered coal deposits in Nigeria has hampered the prospective utilisation for power generation. Consequently, this study is aimed at characterising the physicochemical and thermokinetic properties of Shankodi-Jangwa (SKJ) coal recently discovered in Nassarawa state, Nigeria. The results indicate that SKJ comprises 40.50% fixed carbon, 43.34% volatile matter, and 2.36% sulphur with a higher heating value (HHV) of 27.37 MJ kg. Based on this HHV, SKJ was classified as high-volatile B bituminous coal. Thermal analysis of SKJ under oxidative thermogravimetry (TG) at multiple heating rates revealed that SKJ is highly reactive and thermally degradable below 1000°C. Kinetic analysis using the Flynn-Wall-Ozawa model for conversions α = 0.05–0.90 revealed the activation energy to range from Ea = 113–259 kJ mol, with the frequency factor ranging from A = 2.9 × 10–1.5 × 10 min and a range in R = 0.8536–0.9997; the average values of these ranges are Ea = 184 kJ mol, A = 9.2 × 10 min and R = 0.9420, respectively. The study highlighted fuel property data vital for modelling and designing future SKJ coal power generation.

alumina crucible from 35°C-1000°C at β = 10, 20, 30°C min -1 under an ultrapure oxygen (O 2 ) purge gas flow rate of 20 ml min -1 .Subsequently, the resulting thermograms were analysed using the Pyris 6 TGA software to determine oxidative temperature profiles of SKJ.Next, the parameters of activation energy, E a , and frequency factor, A, were deduced using the Flynn-Wall-Ozawa kinetic model for conversion α = 0.05 to 0.90.

Kinetic Model Theory
The thermal decomposition of SKJ coal under combustion (oxidative) conditions can be represented by the general equation: where α represents the degree of conversion, t represents time, k(T) is the rate constant dependent on temperature, T is absolute temperature, and f(α) is the function of the reaction mechanism occurring during thermal degradation of the material.Consequently, the degree of conversion, α, can be expressed as: 18,19 where m i represents the initial sample mass, m t is the sample mass at time t, and m ∞ is the final sample mass at the end of the reaction.According to the Arrhenius equation, the temperature dependent rate constant, k(T), can be defined as: where A is the frequency factor (min -1 ), E a is activation energy (kJ mol -1 ), R is the universal gas constant (J mol -1 K -1 ) and T is absolute temperature (K), respectively.Consequently, the rate of sample degradation and the effect of the rate-dependent constant on the mechanism of reaction can be obtained by substituting Equation 3 into Equation 1 as given by: By considering and introducing the effect of the heating rate, β, defined as: The thermal degradation of SKJ coal sample can be represented by the equation: After separation of the variables, Equation 6 can be expressed as: By integrating Equation 7, the conversion function, g(α), which describes the thermokinetic decomposition of the SKJ coal at a specific heating rate, can be expressed as: This is the fundamental equation for analysing the parameters of decomposition kinetics; activation energy, E a , and the frequency factor of materials, A. By introducing the Doyle's approximation, 20 the solution to Equation 8 can be deduced, thereby presenting the basis for the isoconversional Flynn-Wall-Ozawa kinetic model given by: Hence, the kinetic parameters E a and A can be deduced by plotting In (β) against (1/T).The E a can be calculated from the slope -1.052 E a /R (where R = 8.314 J mol -1 K -1 ), while A can be calculated from the intercept In [AR/E a ].The higher heating value (HHV) is the most important property for the classification (rank) and assessment of the potential of coals. 21The HHV for SKJ coal is 27.37 MJ kg -1 , which is slightly higher than the value of 27.22 MJ kg -1 that has been reported in literature 3,23 but lower than other Nigerian coals such as Lafia-Obi (30.30MJ kg -1 ), Enugu (32.90 MJ kg -1 ) and Okaba (29.70 MJ kg -1 ). 24n addition, based on HHV and VM, 21 SKJ can be classified as high-volatile B bituminous agglomerating coal.

Thermogravimetric (TG) Analysis
Figure 1 presents the burning profile (oxidative thermal) of SKJ coal at different heating rates.The burning profile of coal is vital in assessing its reactivity, combustibility and suitability for combustion systems. 25The plots clearly displayed the reverse S -weight loss curves typically observed for thermally decomposing carbonaceous materials under non-isothermal conditions. 26,27gure 1: TG plots for SKJ Coal at different heating rates.
The TG plots observably shifted to the right hand side (higher temperatures) due to the thermal-time lag which occurs during TGA at different heating rates.Consequently, the heat transfer and reaction time is limited at higher heating rates, causing the shift in TG curve and temperature profiles. 28Hence, the results demonstrate that the change in heating rate influenced the weight loss of SKJ during oxidative conditions.The DTG plots for SKJ combustion in Figure 2 revealed the typical endothermic peaks for the derivative weight loss of decomposing materials during TGA. 26,27gure 2: DTG plots for SKJ coal at different heating rates.
Similarly, the effect of heating rate was also observed in the DTG plots for SKJ coal.This indicates that the varying heating rate resulted in an increase in the size and orientation of the DTG plots, which highlights the influence of temperature on SKJ coal degradation.Furthermore, the plots also revealed two endothermic peaks for the degradation of SKJ at 10 and 20°C min -1 as was also reported for other Nigerian coals. 25However, the DTG plot at 30°C min -1 indicated two major peaks and one minor peak, which may indicate a higher rate of reactivity of SKJ.
The weight loss peaks for SKJ coal from 30°C-200°C can be ascribed to drying (loss of moisture and mineral hydrates) during thermal degradation. 29The weight loss observed during the drying of SKJ coal ranged from 5.95%-6.65%,which is in good agreement with the determined moisture content (5.05%) for SKJ coal presented in Table 1.Moisture can significantly influence coal classification, processing and thermal efficiency during conversion. 21e weight loss observed for SKJ from 200°C-600°C can be attributed to the devolatilisation of organic matter.The weight loss observed during this stage ranged from 85.95%-86.34%,which suggests that weight loss may not be due only to devolatilisation (as the loss of volatile matter, VM, was only 43.34%) but also to the presence of other components in the coal composition.The combustibility of SKJ was evaluated from the peak decomposition temperature, T max , of the DTG plots.The T max is the temperature at which maximum weight loss occurs and denotes the ease of ignition, reactivity and coal rank; a lower T max indicates a higher rank and thus greater ease of burning or coal degradation. 25,29,30The T max for SKJ ranged from 387°C-400°C from 10°C-30°C min -1 , which is similar to values of 384-451°C reported for Indonesian coals. 31owever, Sonibare and co-workers reported T max values of 445°C-500°C for lignite and sub-bituminous Nigerian coals, 25 which confirms the higher bituminous rank of SKJ.

Combustion Kinetic Analysis
The FWO model was used to determine the activation energy, E a , and frequency factor, A, of SKJ coal combustion.The E a and A were obtained from the slope and intercept of the plot of In (β) against (1/T) at multiple heating rates.Figure 3 presents the kinetic plots for SKJ combustion for conversions α = 0.05-0.90.The values for E a and A for SKJ coal conversion are presented in Table 2.The E a values ranged from 113.13-259.12kJ mol -1 , while A ranged from 2.89 × 10 13 to 1.49 × 10 23 min -1 with correlation values of R 2 = 0.8536-0.9997.The ave 0.9420, reported reported 10 1 -6.7 differen the bitum
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