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THE DETERMINATION OF THE SELF-HEATING TEMPERATURE OF COAL BY MEANS OF TGA ANALYSIS Jale Naktiyok Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02296 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017
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Energy & Fuels
THE DETERMINATION OF THE SELF-HEATING TEMPERATURE OF COAL BY MEANS OF TGA ANALYSIS Jale Naktiyok Department of Chemical Engineering, Atatürk University, 25240 Erzurum, Turkey Abstract In this study, the self-heating tendency of Tavşanlı (Turkey) coal was investigated by the apparatus of thermal analysis. TG-DTG and DSC-DDSC plots obtained at air atmosphere from 25°C to 800°C were used to determine the self-heating temperature (Tsh) and the combustion behavior of coal. It was examined the self-heating temperature changing versus the particle sizes of coal and heating rates, too. Also, kinetic parameters of the main combustion region were obtained in the temperature range of 25°C to 800°C at different heating rates (1, 2.5, 5, and 10°Cmin-1) under non-isothermal heating conditions at air atmosphere. The activation energy, pre-exponential factor and reaction mechanism were calculated by using KAS, FWO, Coats-Redfern, and Master-Plot method. It was understood that the oxidation process of coal was controlled by the first order [F1: -ln (1-α)] mechanism.
Keywords: Self-heating, Combustion profile, KAS, FWO, Coats-Redfern, Master-plot
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1. INTRODUCTION The process of the self-heating or spontaneous combustion is a result of the temperature rise in the mass of the coal or other combustible material. This natural process is the chemical reaction between the oxygen and the coal without any external fire in the place that the oxygen and coal meets (i.e. underground, coal stocks, open mines, and transportation). As a result of this, the temperature of coal starts to increase due to the exothermic internal reactions. If the released heat isn’t removed, it occurs the low temperature oxidation of the coal. When the precaution isn’t taken, the accumulating heat causes the open and uncontrolled fire. The accidents of the coal spontaneous combustion are encountered generally in the coal mining process. It damages the coal resources and leads the energy losses and the disclosure of the poisonous gases that threatens the health of the workers and also it brings about the dust explosion events.1-4 Coals are not prone to the self-heating in the same rate, because the structural properties of the coals, which greatly affect the oxidation process, are different from each other. As described in the literature, the self-heating depends on various factors such as the environmental conditions and the chemical and physical properties of the coal. In particular, the rank of coal effects on the self-heating. Low rank coals (lignite and sub-bituminous coal) are more eager to the self-heating, but the high rank coals (anthracite) is less.2,5-7 It is very important to determine and classify the coals according to this susceptibility in the coal mine.8 There are numerous studies and methods related to the self-heating or spontaneous combustion of coals. Many methods are used to forecast and determine the self-heating tendency (Tsh) of coal.9-21The thermal analysis method TG-DTG/DSC is used in characterization of the fossil fuels especially undergoing combustion or pyrolysis reactions because of their fast and cheap analysis and easy performance of experiments.16-19, 22-26 It can
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provide the opportunity to the determination of the self-heating tendency of coal instead of traditional methods. The satisfactory results are obtained from this method in recent studies.2024,27-30
The DSC analysis displays the reactions taking place during the oxidation process. It is not easy to select a specific reaction and accurately to determine the initial temperatures of the reactions because of the quite complex nature of the coal oxidation at low and high temperatures. However, the first derivative (DDSC) of the DSC data enables in determining the initial temperatures of numerous and different reactions.23,29 The aim of this study tries to determine the self-heating temperature and the kinetics terms of the Tavşanlı-coal by TGA-DTG/DSC methods and to provide widespread utilization of the technique. Kinetics Analysis TG analysis identifies the mechanism of the physical and chemical processes occurring during coal combustion. In this present study, it was applied model-free techniques (KAS, FWO) and the model-fitting technique (Coast-Redfern) and Master-Plot method for the kinetics analysis at non-isothermal decomposition conditions of coal.25-40 It is given the required formulas and explanation for KAS, FWO, Coast-Redfern techniques in previous study.22 But it will be informed about only the Master plot method here. In the kinetic analysis, it is very important the knowledge of the dependence of activation energy (E) with the conversion value of the decomposition (α). It provides to explain multistep processes and gives information about their reaction mechanisms. When the rate of a solid-state decomposition reaction under non-isothermal condition is determined, α is firstly calculated as follows: =
− (1) −
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W0, Wt, and W∞ are the initial, actual, and final mass of the sample, respectively. The reaction-rate of a material is usually explained as; = () (2) f(α) is a function related to the reaction mechanism and k is the specific rate constant at the Arrhenius equation: = exp (−
) (3)
A is the pre-exponential factor (min-1), Ea is the activation energy (kJmol-1), R is the universal gas constant (Jmol-1K-1), and T is the absolute temperature (K). is the constant heating rate for the non-isothermal system. = (4) From Eq. 2, it is obtained = . = ( ) (5) When above equations are rearranged, it is found Eq. (6) E! = . exp − . (6) () For low temperatures, the reaction-rate is considered to be small enough to be neglected and if the both sides of Eq. 6 is integrated from 0 to α and from 0 to T, it is obtained Eq. (7). # ( ) = $
'
& E! = ( ) $ exp − (7) ( )
#( ) is a new function, called as the temperature integral and haven’t an analytical solution. If ( =
)*
&
)
, then the expression of , exp -− +&* . reduces to +&
)* +
/ 01
,2
23
)
( = - +* . 4(().
2 5 62 4(() = $ − 7 ( (8) (
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4(() is an infinite function of (. Substituting the values of temperature integral (4(()) in Eq. 7 gives: # ( ) = $
'
AE! = 4(() (9) ()
This equation is dependent on evaluating the function 4(( ). Doyle has evaluated 4(() and suggested.41 4(() = 0.00484exp (−1.0516)
(10)
The Master-plot method provides the solution of Eq. (9) by using a reference at point α=0.5, and then the following Eq. (11) is found. #() = 4((.< ) (11) (.< = /.< . Eq. (12) is obtained by dividing Eq. (10) to Eq. (11) #() 4(() = (12) #(0.5) 4((.< ) The curves of
>(?)
>(.
