Formulation of the Heat Generation Rate of Low-Temperature

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Formulation of heat generation rate of low temperature oxidation of coal by measuring heat flow and weight change at constant temperatures using TG-DSC Kouichi Miura, Hideaki Ohgaki, Nobuyuki Sato, and Masaharu Matsumoto Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01817 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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Formulation of heat generation rate of low temperature oxidation of coal by measuring heat flow and weight change at constant temperatures using TGDSC

Kouichi Miura*, Hideaki Ohgaki, Nobuyuki Sato1), and Masaharu Matsumoto1)

Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan 1) Electric Power Development Co.,Ltd,Chigasaki 253-0041, Japan

*) Telephone: +81-774-38-3420. Fax: +81-774-38-3426. Email: [email protected]

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Abstract

This work successfully measured heat generation rates accompanying the air oxidation of 3 low rank coals at 7 constant temperatures ranging from 50 to 150 °C using a TG-DSC analyzer. The heat generation rates were well expressed by the sum of 3 parallel first order reactions which are expressed by

dQj dt

= k jQj0 exp(−k jt ) ( j = 1, 2, and 3)

where Qj is the amount of heat generated, t is the time, kj is the first order rate constant, Qj0 is the maximum amount of heat generated, and kjQj0 gives the maximum heat generation rate. The rate constants k1, k2, and k3 were well represented by the Arrhenius equations. The activation energies were 2.78 to 7.58 kJ/mol for k1, 9.29 to 13.48 kJ/mol for k2, and 55.5 kJ/mol for k3, respectively. The maximum amounts of heat generated by the reaction 3, Q30, were constants irrespective of temperature ranging from 340 to 671 kJ/kg-coal as expected. On the other hand, Q10 and Q20 had to be regarded as variables increasing with temperature. The calculated dQ/dt vs. t relationships and Q vs. t relationships using the estimated kj and Qj0 showed excellent agreements at all temperatures for all of the coals. This shows that the rate parameters obtained can well be used to represent the heat generation rate at temperatures between 50 to 150 °C. It was also found that H2O forming reaction with simultaneous formation of coal-oxygen complexes is the dominant process of oxygen-coal interaction in the temperature range examined based on the measurements of weight changes and formation rates of H2O, CO2, and CO.

Keywords Heat generation rate equation, kinetic analysis of low temperature oxidation, spontaneous heating of coal, self-ignition of coal, low rank coal,


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1.

INTRODUCTION Low-rank coals, which are abundantly deposited and widely distributed, are important

resources in this century to be utilized effectively around the world. Their high self-ignition tendency, especially when dewatered, however, has made their transportation and storage extremely difficult. This has limited their use to fuel for power plants near mining sites. It is, therefore, essential to develop the methods suppressing the self-ignition of low-rank coals based on the understanding of the mechanism of spontaneous heating and self-ignition. Spontaneous heating has been the subject of research for more than 100 years, and a huge number of works have been published. In 1979, Kim1 smartly summarized the factors affecting spontaneous heating, indices of combustibility, and experimental methods with 45 pertinent references. In 1994, Carras and Young2 published an excellent review on models, application and test methods on spontaneous heating of coal and related materials in stockpile. In 2003, Wang et al.3 reviewed coal oxidation at low temperatures mainly focusing on the chemical reactions and the kinetic modelling. In 2005, Nelson and Chen4 published a comprehensive review of 53 pages with pertinent 200 literatures, which examined experimental works covering the period of 1996− 2005. The review of Carras and Young, for example, summarized the essence of the spontaneous heating and self-ignition in stockpiles very clearly. They described that spontaneous heating of coal stockpile occurs when the rate of heat generation within the stockpile is greater than the rate at which heat can be transported to and dissipated in the external environment. Air transported into the stockpile provides oxygen for the oxidation of coal. The heat liberated is transported into and away from the stockpile. Water vapor is transported either into the stockpile or away from the stockpile depending on the relative humidity within the coal. Outdoor stockpiles are also affected by the weather through wind, rain, and solar radiation. To develop a general quantitative model for spontaneous heating of stockpile, quantitative description of each of the heat generation and dissipation processes is essential. If we focus on the heat generation process, most of heat is generated through the interaction of oxygen and coal. The interaction between water vapor and coal also generates significant amount of heat when water condenses on dried coal. These two processes must be taken into account in any attempt to describe the self-heating of coal stockpile as was stressed by Carras and Young. In a previous work Miura has shown that the rate of adsorption of water vapor on dried coal is very rapid even from the ambient atmosphere and generates large amount of heat


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and hence adsorption of water vapor will contribute to raise the coal temperature significantly5,6. The coal heated by the adsorption of water vapor, however, needs to be heated further through the interaction with oxygen if it is finally led to self-ignition. It is, therefore, essential to investigate both the mechanism and kinetics of the interaction of oxygen and coal as has been performed by many researchers. The mechanism of low-temperature oxidation of coals have been investigated using Fourier transform infrared spectroscopy (FTIR)7,8, X-ray photoelectron spectroscopy (XPS)9,10 and gas chromatography. It is generally accepted that the aliphatic carbon moieties are main reactants of the oxidation at low temperatures. The aliphatic carbons are first oxidized to form peroxides,11,12 and then the peroxides are further oxidized to form oxygen-containing functional groups, such as aldehyde, carboxyl, esters, and anhydrides, with simultaneous formation of H2O and CO2. The aliphatic carbon moieties are finally converted to CO2, H2O, and CO gases. Oxidation of the aliphatic carbon moieties will generate heat, raise the coal temperature, accelerate the oxidation reaction, and finally ignite coal, leading to self-ignition. Quantitative and kinetic examinations of the low-temperature oxidation process are essential to formulate rate equation that can express the heat generation rate as the functions of coal conversion, oxygen pressure, and temperature based on the proposed oxidation mechanism. The above cited extensive review by Wang et al.3 summarizes the kinetic parameters reported in the literature before 2002. The present state of the kinetic analysis of coal-oxygen interaction at low temperature is, for example, smartly summarized by Slovak and Taraba13 as follows: “Process of low-temperature coal oxidation involves a large number of individual reactions running at the same temperature range, and their kinetic parameters are not experimentally available (at present state of our knowledge). In addition, the situation is complicated by the fact that the coal–oxygen interaction leads to both gaseous and solid oxidation products, which both affect measured mass changes. The only parameters, we are able to determine, describe the process as whole and therefore have to be understood as not ‘‘true’’ but ‘‘observed’’ or ‘‘effective.’’ Usability of them to predictions or mechanistic speculations is limited but they characterize dynamics of the whole oxidation process and can be used as characteristics of coal oxidation behavior (e.g., activation energy as the measure of the dependence of oxidation rate on temperature).”


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Recent works14-17 on kinetic examinations of the low-temperature oxidation process do not completely answer the comments raised by Slovak and Taraba. The authors18 also have examined changes of weight, gas formation rates, heat flow, and functional groups of coal during non-isothermal oxidation at a constant heating rate using a TG analyzer connected to a micro gaschromatograph, a sensitive DSC equipment, and in situ FTIR to identify the reactions and to formulate the heat generation rate. However, the changes of the properties except the heat flow were too small below 100 °C to discuss the mechanism and kinetics of oxygen-coal interaction. DSC was believed to be sensitive enough to detect the heat flow even at the low temperature, but the measured heat flow of this low temperature range was endotherm, because the sensible heat (endotherm) to raise the coal temperature was larger than the heat generated by the oxygen-coal interaction. This suggested that the non-isothermal oxidation technique is not appropriate to examine the oxygen-coal interaction at low temperatures, although the technique has been widely used in the literature19-22. On the contrary, isothermal oxidation experiments performed by Clemens et al. 23 using a DTA clearly detected an immediate sharp exotherm even at 30°C when oxygen contacted dried coals.

At all temperatures the coal least prone to self-ignition has a significantly smaller

response than the other coals. The exotherm increases with increasing temperature. Since the isothermal oxidation experiment does not include the contribution of sensible heat, the measured exotherm is expected to be directly related to the heat generated by the coal-oxygen interaction. Unfortunately, however, the DTA response was given as the unit of µV/g-coal in the work of Clemens et al. and could not be directly converted to heat flow.

Recent sensitive DSC

equipment can well overcome the weakness of DTA and hence is expected to be utilized to measure the heat generated by the oxygen-coal interaction at constant low temperatures. Then the purposes of this work are to examine the oxygen-coal interaction at 7 constant temperatures of 50, 70, 90, 107, 120, 135, and 150 °C based on simultaneous measurements of weight and heat flow using a TG-DSC analyzer and to formulate rate equations that can express the heat generation rates in the temperature range. Supporting experiments measuring gas formation rates using a sensitive micro gaschromatograph and in situ FTIR measurements are also undertaken at several conditions.

2. EXPERIMENTAL SECTION


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2.1 Coal Samples used. Table 1 lists the properties of three coals used. Coal A and coal S1 are Indonesian lignites, and coal C is an Australian subbituminous coal. As-received coals were ground to less than 150 µm in diameter and stored in airtight bags to prevent the oxidation in ambient atmosphere. The airtight bags were opened just before the experiment and the remaining coals were transferred to airtight bin and stored in a refrigerator. 2.2 Measurement of weight change and heat flow during oxidation of coal. A TG-DSC analyzer (NETZSCH, STA 449F3) was used for simultaneous measurements of weight change and heat flow during the oxidation of coal at constant temperatures. Figure 1 shows typical data obtained from the measurement. About 15 mg (on dry basis) of coal sample placed in a PtRh crucible (6.8 mmφ x 3 mmH) was set on the sample probe and an identical empty PtRh crucible was set on the reference probe. The sample crucible and the reference crucible were heated at the rate of 20 K/min to 107 °C at which they were kept 15 min in a 100 mL/min of N2 stream to evaporate water adsorbed on coal. Large weight decrease in Fig. 1 shows that coal A contains more than 25 % of inherent water. The large endothermic heat flow was detected accompanying the weight decrease. Then the sample crucible and the reference crucible were heated or cooled at the rate of 10 K/min to reach a constant reaction temperature (150 °C in Fig. 1) and kept for 20 min to stabilize the temperature, after that the gas stream was changed to 100 mL/min of dry air stream to start the oxidation experiment. Small but clear exothermic heat flow was detected with small increase of weight. After 30 min of oxidation, the gas stream was switched back to N2 to terminate the oxidation and to check the base line of heat flow. The weight change and heat flow shown in Fig. 1 are corrected ones using the so called “correction run” that measures weight and heat flow using the empty sample crucible under exactly same conditions.

The oxidation

experiments were performed at 7 constant reaction temperatures of 50，70，90，107，120， 135，and 150 °C. 2.3 Measurement of gas formation during oxidation of coal. The gas formation rates of H2O, CO2, and CO were measured to examine the chemical reactions taking place accompanying the weight change and heat flow at several reaction temperatures. To do so, about 150 mg (on dry basis) of coal sample was packed in a glass tube of 6 mm in ID and it was heated under the same conditions used for the TG-DSC measurement except that He and 22% oxygen-78% He mixture were used instead of N2 and air.

The exit gas stream was directly connected to a micro


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gaschromatograph (Varian, CP4900) with a MS5A column and a Porapack Q column to quantify CO, CO2, and H2O in every 70 s. 2.4. Observation of the changes of Functional Groups in Coal. Changes of functional groups in coal through oxidation were observed at several reaction temperatures using in situ FTIR method with a spectrometer (JEOL, JIR-WINSPEC50) equipped with a hot stage (JEOL,CS-30) and a microscope (JEOL, MAV 204). About 0.5 mg of sample was leveled and pressed on a sample holder in the hot stage using a spatula. The heating profile and the gas stream exactly followed the TG-DSC experiment except that the gas flow rates were set to be 50 mL/min. The diffuse reflectance spectra were collected by acquisition of 64 scans at 4 cm−1 of resolution at 2 to 3 min of intervals. The infrared (IR) spectra were converted to the Kubelka−Munk (K−M) function.

3. RESULTS AND DISCUSSION 3.1 Weight change and heat generation during the oxidation. Figures 2a to 2c show the weight changes measured during the oxidation at 7 constant temperatures for the three coals. The weight changes are shown as weights relative to dry coals using same scale to facilitate the comparison among the coals. The relative weights increased at all temperatures with increasing time for all of the coals, but reached as small as 1.008 even at the highest temperature of 150 °C for coal C. The weight changes were so small at 50 °C for all of the coals. Figures 3a to 3c show the heat generating rates, dQ/dt, which were directly and simultaneously measured with the weight changes, and the accumulated amounts of heat generated, Q, which were obtained by integrating dQ/dt over time, for the three coals. Again all figures are shown using the same scale to facilitate the comparison among the coals. The dQ/dt values were successfully measured at all temperatures. The dQ/dt and Q values clearly indicate that appreciable heat is generated even at 50 °C. The dQ/dt values increased with temperature and decreased with the elapse of oxidation time as was observed by Clemens et al.23 They reached as high as 0.5 kW/kg for both coal A and S1 at 150 °C. The Q values after 30 min of oxidation at 150 °C reached as large as 350 kJ/kg for coal A and S1. 350 kJ/kg of Q value is more than enough to increase the coal temperature by over 250 °C under adiabatic conditions. It is worthy to note that coal C, which showed largest weight increase at 150 °C, gives smallest


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heat generation of the three coals. This suggests that weight change solely cannot be an index to estimate the propensity to spontaneous heating of coal. 3.2 Gas formation and change in oxygen functional groups during the oxidation. Figure 4 shows gas formation rates measured at three temperature levels of 70, 107, and 135 °C for coal A and at 107 °C for coal C. First, let us focus on coal A. Note that the abscissas are doubled with increasing temperature. At all temperatures H2O is the dominant gas species formed. Next abundant gas species formed was CO2, but its formation rate was much smaller than that of H2O even at 135 °C. Formation of CO was negligible at 70 °C, but small formation was observed at 135 °C. For coal C, H2O is again the dominant gas species formed but its formation rate is much smaller than that for coal A at 107 °C. Formation of CO was negligible even at 107 °C for coal C. Gas formation behaviors are different among the coals by reflecting the weight change and heat flow. Wang et al.24 reported that the molar formation ratio among H2O, CO2, and CO at constant temperatures of 60 to 90 °C is about 21:3:1 from the constant temperature oxidation using around 150 g of coal sample. Our results well agree to the work of Wang et al. Several mechanisms have proposed that the initial stage of low temperature oxidation is intake of oxygen as peroxides10,

11, 18, 23, 24-27

. Clemens et al.23, for example, stated that the

oxidation reactions involve the exothermic formation of hydroperoxides followed by their decomposition into carboxylic acid/aldehyde species. The latter process produces H2O. The data of gas formations together with the weight change data allow us to estimate the net amount of oxygen uptake in coal during the oxidation, nO, from the oxygen balance given by nO = (Weight increase measured by TG-DSC) + (Amounts of C and H involved in formed gases) This equation assumes that N and/or S containing gases are not formed during the oxidation. The value of nO will be closely related to the amount of hydroperoxides formed. Then the changes of nO vs. time relationships were calculated by using the gas formation results shown in Fig. 4 and the corresponding weight change data, and they are shown in Figure 5. The effects of temperature can be examined using the results for coal A. At 70 °C nO is small and slightly larger than the corresponding weight increase, indicating that the weight increase is nearly equal to the amount of oxygen uptake, in other words, the amounts of gas formed are very small. The value of nO is larger than the corresponding weight increase at 107 °C and


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significantly larger than the corresponding weight increase at 135 °C, indicating that the gas formation reaction becomes significant with increasing temperature. This changing trend of nO well supports the oxidation mechanism introduced above. At 107 °C coal A shows larger nO and weight increase than coal C, indicating that coal A is more reactive than coal C. The above examinations show that the heat is generated by taking in oxygen from gas phase, simultaneously producing mainly H2O by the oxidation of coal. The amount of net oxygen uptake, nO, and the amount of H2O formed were, for example, respectively 0.0092 kg/kg and 0.73 mol/kg after 30 min of oxidation at 135 °C for coal A. The nO value of 0.0092 kg/kg-coal is much smaller than O content of coal (20.6 % on d.a.f. basis). 0.73 mol/kg-coal of H2O means that 0.00146 kg/kg-coal of H2 was evolved as gaseous H2O, which means less than 3 % of H2 in the coal was consumed to form H2O. These small changes generate the heat as large as 215 kJ/kg-coal which accounts for 7.5 % of heating value of coal A. To observe directly the changes in oxygen functional groups of coal during the oxidation, in situ FTIR spectra were measured under several conditions. Figure 6, as an example, shows the spectra measured for coal A at 135 °C. These changes in FTIR spectra correspond to the changes in the weight, gas formation rate, and nO which were just discussed above. Unfortunately we could not detect appreciable changes in the spectra during 30 min of oxidation even at as high as 135 °C. This is because the changes in the functional groups were too small to be detected by the in situ FTIR technique as can be anticipated from the above discussion. Thus we were unable to confirm the oxidation mechanism from the in situ FTIR measurement. Only the gas formation and accompanying heat generation behaviors support the oxidation mechanism discussed above. 3.3 Definition of rate equation of low temperature oxidation. Slovak and Taraba13 stated that process of low-temperature coal oxidation involves a large number of individual reactions running at the same temperature range, as was introduced in Introduction. Clemens et al.19 also stated that the oxidation reactions involve the exothermic formation of hydroperoxides followed by their decomposition into carboxylic acid/aldehyde species. The weight change and gas formation results discussed above also suggest that the oxidation reaction consists of complex reactions involving consecutive process. The dQ/dt vs. t relationships measured, however, were judged to express the complex changes with good accuracy as shown in Fig. 3. Then it was intended to formulate the heat generation rate by using the dQ/dt vs. t relationships measured at 7 constant temperatures.


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We have tried to fit the dQ/dt vs. t relationship by several single reaction models, including the Elovich equation that is widely used for the chemisorption as preliminary examinations. It was also easily seen from ln(dQ/dt) vs. t relationships that neither single first order reaction nor reactions with the reaction order greater than 1 fit the data. In short, no single reaction models that we tested could fit the dQ/dt vs. t relationships. Then we assumed as a first approximation that the low temperature oxidation of coal consists of several parallel first order reactions. Then the rate of the j-th reaction can be expresses as

−

dn j dt

= k j (T , pO2 )n j

(1)

where nj is the amount of the j-th species remaining and k j (T , pO 2 ) is the first order rate constant of the j-th reaction which is a function of temperature, T, and oxygen pressure, p O 2 . Since k j (T , pO 2 ) is a constant at a constant temperature and in dry air, it is simply written as kj

hereinafter. Using the enthalpy of the j-th reaction, ∆HR,j(T), the heat generation rate by the j-th reaction, dQj/dt, is written as follows:  dn  = (− ∆H R, j (T ) ) − j   dt  dt  

dQ j

(2)

The accumulated amount of heat generated by the j-th reaction, Qj, is represented as follows:

Q j = (− ∆H R, j (T ) )(n j0 − n j ) = − ∆H R, j (T )n j0 + ∆H R, j (T )n j = Q j0 + ∆H R, j (T )n j

(3)

where Qj0 is the maximum amount of heat generated by the j-th reaction. Combining Eqs. (1) to (3) gives

dQj dt

= k j (Qj0 − Qj )

(4)

This equation is the first order rate equation that uses Qj as dependent variable, and it can be integrated to give Q j = Q j0 {1 − exp( − k jt )}

(5)

Inserting Eq. (5) into Eq. (4) gives dQj/dt as follows:

dQj dt

= k jQj0 exp(−k jt )

(6)


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Since the total heat generation rate, dQ/dt, is the sum of the contributions of all reactions (

= ∑ (dQ j / dt ) ), the kinetic analysis is to find the number of reactions, N, and to estimate kj and N

j=1

Qj0 of each reaction.

3.4 Kinetic Analysis to formulate heat generation rate. Figure 7 schematically shows the method employed to estimate, N, kj and Qj0 from dQ/dt vs. t relationship measured at a constant temperature. (1) Experimentally obtained dQ/dt vs. t relationship was plotted as ln(dQ/dt) vs. t relationship (green solid curve). If the reaction at this temperature can be represented by a single reaction, the ln(dQ/dt) vs. t relationship can be represented by a straight line as Eq. (6) indicates. However, it is not the case, indicating that the reaction consists of several reactions. (2) Assume that one of the reactions can be represented by the black broken line represented tentatively as  dQ  ln 3  = ln(k3Q30 ) − k3t  dt   

(7)

k3 and Q30 are estimated from the slope and the intercept of the line. (3) The contributions of the reactions other than reaction 3 can be calculated by subtracting the black broken line from the green solid line to give the blue solid line. Since the blue solid line cannot be represented by a straight line, the blue line consists of contributions of several reactions. Then we assume that the next reaction, reaction 2, can be represented by the blue broken line as  dQ  ln 2  = ln(k2Q20 ) − k2t  dt   

(8)

k2 and Q20 are estimated from the slope and the intercept of the line. (3) The contributions of the reactions other than reactions 3 and 2 can be calculated by subtracting the blue broken line from the blue solid line to give the red solid line. The red solid line could be well approximated by a straight line (the red broken line) given by  dQ  ln 1  = ln(k1Q10 ) − k1t  dt   

(9)

k1 and Q10 are estimated from the slope and the intercept of the red broken line.


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The good agreement between the experimentally obtained ln(dQ/dt) vs. t relationship and the calculated ln(dQ/dt) vs. t relationship (the green broken line) shows that the reaction can be approximated by three parallel first order reactions. The number of reactions, N, could be equated to 3 for all of the coals at all temperature levels. By analyzing all the dQ/dt vs. t relationships for the three coals kj and Qj0 were estimated and are listed in Table 2. kj and kjQj0 are also shown as the Arrhenius type plots in Figures 8a and 8b, respectively. kjQj0 gives the maximum heat generation rate of each reaction as Eq. (6) shows. The first order rate constants k1, k2, and k3 were well represented by the Arrhenius equation for the three coals. The activation energies were 2.78 to 7.58 kJ/mol for reaction 1, 9.29 to 13.48 kJ/mol for reaction 2, and 55.5 kJ/mol for reaction 3, respectively. The maximum amounts of heat generated by the reaction 3, Q30 were regarded as constants irrespective of temperature ranging from 340 to 671 kJ/kg as expected. On the other hand, Q10 and Q20 extracted by the procedure shown in Fig. 7 could not be set to be temperature independent constants but variables increasing with temperature as listed in Table 2. The Q10 value, for example, was estimated to be as small as 1.59 kJ/kg-coal at 50 °C, 9.25 kJ/kg-coal at 107 °C, and as large as 25.6 kJ/kg-coal at 150 °C for coal A. Temperature dependant Qj0 may be unusual as rate equation and another reaction scheme may be more suitable to express the low temperature oxidation.

However, the calculated dQ/dt vs. t

relationships and Q vs. t relationships using the estimated kj and Qj0 showed excellent agreements at all temperatures for all of the coals as shown as the broken lines in Figs. 3a to 3c. This may not confirm the soundness of the kinetic analysis, but the rate parameters obtained can well be used to represent the heat generation rate at temperatures between 50 to 150 °C. The estimated maximum heat generation rate ( = ∑ (k jQj0 ) ) shown by the thick solid lines in Fig. 8b are 3

j =1

approximated by two Arrhenius equations, for example, having E = 36 kJ/mol for temperatures lower than 90 °C and having E = 48 kJ/mol for temperatures higher than 90 °C for coal A. We tried to compare the activation energies obtained in this work with those published in the literature. Many attempts have been performed to estimate activation energies of low temperature oxidation of coal.

Wang et al.3 summarized those values in the literature

systematically with experimental method employed. The work performed by Banerjee et al.28 measured oxygen consumption rates using a flow type reactor at several constant temperatures between 30 to 168 °C for 7 low rank coals. By assuming that the overall reaction is first order


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with respect to oxygen concentration, they estimated the activation energies as 3.9 to 5.7 kcal/mol at 50 °C, 8.2 to 13.1 kcal/mol at 70 °C, and 11.3 to 13.1 kcal/mol at 100 °C. These values are rather close to the overall activation energies of maximum amounts of heat generated ( = ∑ (k jQj0 ) ) shown in Fig. 8b. 3

j =1

The contributions of three reactions calculated using the estimated kj and Qj0 at several conditions are schematically shown in Figures 9a to 9c. Figs 9a and 9b compare the dQj/dt vs. t and Qj vs. t relationships between coal A and coal C at 150 °C. The reaction 1 is completed in 5 min or so, and the reaction 2 is completed in 20 min or so, but the reaction 3 is completed by only 37 % even after 30min of oxidation. Figs 9a and 9c compare dQj/dt vs. t and Qj vs. t relationships at 150 °C and 70 °C for coal A. The reaction 1 is completed in 7 min or so even at 70 °C as the small activation energy for this reaction indicates. The contributions of both reactions 1 and 2 are dominant at the initial stage of oxidation at 70 °C. These reactions generate 14 kJ/kg of heat in 30 min, indicating that the heat generation rate is rather small at 70 °C. The estimated kj and Qj0 allows us to estimate the contributions of three reactions between 50 and 150 °C for the three coals.

3.5 Heat generation rate of weathered coal. Finally it is worthy to note that the activation energy for reaction 1 was as small as 2.78 to 7.58 kJ/mol. Such small activation energies indicate that reaction 1 will proceed at high rate even at ambient temperature as was shown in Fig. 9c for coal A. This suggests that oxidation of as-received coal in ambient atmosphere, that is called weathering of coal, will proceed rather rapidly. To examine the effect of weathering on the oxidation rate, coal A stored in the airtight bag was transferred to a 100 mL bin and kept at room temperature for several months to prepare a weathered coal. Then dQ/dt vs. t relationships were measured at constant temperatures ranging from 30 to 147 °C for the weathered coal, and the heat generation behavior of the weathered coal was compared with that of as-received coal A at three temperature levels as shown in Figure 10. The heat generation rates of the weathered coal were less than half of those of as received coal at all temperatures examined. This clearly shows that the weathered coal had been oxidized significantly during the several months of storage even at room temperature. To examine how weathering affects the rate parameters of coal oxidation the dQ/dt vs. t relationships for the weathered coal were analyzed to estimate the rate parameters as was done


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Energy & Fuels

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Page 14 of 37

for the as received coal. The kj and kjQj0 values estimated are shown with those for the as received coal as the Arrhenius type plots in Figures 11a and 11b, respectively. Three parallel first order reactions well fitted the dQ/dt vs. t relationships for the weathered coal as well. The first order rate constants k1 were rather close each other for the as received coal and the weathered coal, but k2, and k3 of the weathered coal were slightly smaller than those of the as received coal with larger activation energies as shown in Fig. 11a. The maximum rates of heat generation, kjQj0, of the weathered coal were 1/3 to 1/2 of those of the as received coal. As the result, the estimated maximum heat generation rate of the weathered coal (the thick red solid line) was 1/3 to 1/2 of those of the as received coal (the thick black line). In other words, weathering will reduce the degree of propensity to spontaneous heating. The rate equations shown in Table 2, Fig. 8a, and Fig.8b are for the fresh dried coals and dry air as the first stage of analysis. Figs. 11a and 11b, however, show that similar analysis can be applied to weathered coal also. This suggests that the analysis method presented in this work is valid for estimating kinetic parameters in dry air. Of course, involvement of oxygen pressure dependency in the rate equation and the examination of effect of water vapor and/or inherent water on the coal-oxygen interaction are essential to apply the rate equations to actual spontaneous combustion. Such works are in progress in our research group.

4. Conclusions Heat generation rates accompanying the air oxidation at 50 to 150 °C were successfully measured for 3 kinds of low rank coals using a TG-DSC analyzer. The heat generation rates measured were well expressed by 3 parallel first order reactions which are expressed by Eq. (6) as

dQj dt

= k jQj0 exp(−k jt ) ( j = 1, 2, and 3)

(6)

where kj is the first order rate constant of the j-th reaction, Qj0 is the maximum amounts of heat generated by the j-th reaction, and kjQj0 gives the maximum heat generation rate of the j-th reaction. The rate constants k1, k2, and k3 were well represented by the Arrhenius equations for the three coals. The activation energies were 2.78 to 7.58 kJ/mol for k1, 9.29 to 13.48 kJ/mol for k2, and 55.5 kJ/mol for r k3, respectively. The maximum amounts of heat generated by the reaction 3,


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Energy & Fuels

Q30 were regarded as constants irrespective of temperature ranging from 340 to 671 kJ/mol as expected. On the other hand, Q10 and Q20 were variables increasing with temperature. The calculated dQ/dt vs. t relationships and Q vs. t relationships using the estimated kj and Qj0 showed excellent agreements at all temperatures for all of the coals. This shows that the rate parameters obtained can well be used to represent the heat generation rate at temperatures between 50 to 150 °C.

REFERENCES

(1) Kim, A.G. Laboratory studies on spontaneous heating of coal. A summary of information in the literature. U.S. Dept of the Interior, Bureau of Mines Information Circular 8756 1977. (2) Carras, J.N.; Young, B.C. Self-heating of coal and related materials: Models, application and test methods. Prog. Energy Combust. Sci. 1994, 20, 1-15. (3) Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M. Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling. Progress in Energy and Combustion Science 2003, 29, 487-513. (4) Nelson, M.I.; Chen, X.D. Survey of experimental work on the self-heating and spontaneous combustion of coal. 31-83 in Geology of Coal Fires: Case Studies from Around the World. Reviews in Engineering Geology XVIII 2007, Stracher, G.B. (Ed), The Geological Society of America. (5) Miura, K. Measurement of temperature increase of dried coal on exposure to ambient atmosphere. (in Japanese) J. Jpn Inst. Energy 2015, 94, 1169-1172. (6) Miura, K. Adsorption of water vapor from ambient atmosphere onto coal fines leading to spontaneous heating of coal stockpile. Energy Fuels 2016, 30, 219−229. (7) Wang, D.; Zhong, X.; Gu, J.; Qi, X. Changes in active functional groups during lowtemperature oxidation of coal. Min. Sci. Technol. 2010, 20, 35−40. (8) Yürüm,Y.; Altuntas, N. Air oxidation of Beypazari lignite at 50°C, 100 °C and 150 °C. Fuel 1998, 77, 1809−1814. (9) Perry, D. L.; Grint, A. Application of XPS to coal characterization. Fuel 1983, 62, 1510−1512. (10) Kelemen, S. R.; Freund, H. Oxidation kinetics of Wyoming Powder River Basin coal in O2 between 295 and 398 K. Energy Fuels 1990, 4, 165−171.


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(11) Jones, R. E.; Townend,D. T. A. Mechanism of the oxidation of coal. J. Soc. Chem. Eng. Ind. 1949, 68, 197−201. (12) Dack, S. W.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Free radical involvement in the drying and oxidation of Victorian brown coal. Fuel 1984, 63, 39−42. (13) Slovak, V.; Taraba, B. Effect of experimental conditions on parameters derived from TGDSC measurements of low-temperature oxidation of coal. J. Therm. Anal. Carolim. 2010, 101, 641-616. (14) Taraba, B.; Pavelek, Z. Investigation of the spontaneous combustion susceptibility of coal using the pulse flow calorimetric method: 25 years of experience. Fuel 2014, 125, 101−105. (15) Torrent, J. G.; Anez, N. F.; Pejic, L. M.; Mateos, L. M. Assessment of self-ignition risks of solid biofuels by thermal analysis. Fuel 2015, 143, 484−491. (16) Arisoy, A.; Beamish, B. Reaction kinetics of coal oxidation at low temperatures. Fuel 2015, 159, 412−417. (17) Zhao, H.; Geng, X.; Yu, J.; Xin, B.; Yin, F.; Tahmasebi, A. Effects of drying method on self-heating behavior of lignite during low-temperature oxidation. Fuel Process Techn 2016, 151, 11−18. (18) Fujitsuka, H.; Ashida, R.; Kawase, M.; Miura, K. Examination of low-temperature oxidation of low-rank coals, aiming at understanding their self-ignition tendency. Energy Fuels 2014, 28, 2402−2407. (19) Kok, M. V.; Okandan, E.; Kinetic analysis of DSC and thermogravimetric data on combustion of lignite. J. Thermal Analysis 1996, 46, 1657-1669. (20) Ozbas, K. E.; Kok, M. V.; Hisyilmaz, C.; DSC study of the combustion properties of Turkish coals. J. Thermal Analysis Calorim. 2003, 71, 849-856. (21) Malow, M.; Krause, U. The overall activation energy of the exothermic reactions of thermally unstable materials. J. Loss Prev. Process Ind. 2004, 17, 51-58. (22) Li, B.; Chen, G.; Zhang, H.; Sheng, C. Development of non-isothermal TGA-DSC for kinetic analysis of low temperature coal oxidation prior to ignition. Fuel 2014, 118, 385-391. (23) Clemens, A. H.; Matheson, T.W.; Rogers, D. E. Low temperature oxidation studies of dried New Zealand coals. Fuel 1991, 70, 215-221. (24) Wang, H., Dlugogorski, B. Z., Kennedy, E. M., Examination of CO2, CO, and H2O Formation during low-temperature oxidation of a bituminous coal. Energy Fuels 2002, 16, 586-592 (2002).


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Energy & Fuels

(25) Liotta, R.; Brons, G.; Isaacs, J. Oxidative weathering of Illinois No.6 coal. Fuel 1983, 62, 781–791. (26) Kelemen, S. R.: Freund, H., Oxidation kinetics of Illinois No.6 coal in air between 295 and 398 K. Energy Fuels, 1989, 3, 498-505. (27) Wang, H., Dlugogorski, B. Z., Kennedy, E. M., Analysis of the mechanism of the lowtemperature oxidation of coal. Combustion Flame 2003, 134, 107-117. (28)Banerjee, S. C.; Banerjee, B. D.; Chakravorty, R. N., Rate studies of aerial oxidation of coal at low temperatures (30-170 °C). Fuel 1970, 49, 324-331.


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Table 1 Coal

Page 18 of 37

Analyses of the coals used

Ultimate analysis [wt%, d.a.f.]

Proximate analysis [wt%, d.b.]

C

H

N

S

O

FC

VM

Ash

A

73.1

5.1

1.1

0.1

20.6

50.0

48.0

2.0

S1

71.1

5.1

1.0

0.2

22.6

46.4

49.9

3.7

C

77.1

5.4

1.6

0.8

15.1

63.7

29.6

6.7


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Energy & Fuels

Table 2. kj and Qj0 estimated for the three coals k1 Coal

-1

[s ]

Q10

k2

[kJ/kg]

-1

[s ]

Q20

k3

[kJ/kg]

-1

7.81x10 6 e −4800 / T (>90℃)

A

C

S1

0.105e −7580 / RT

0.0338e

0.126e

−2780 / RT

−7580 / RT

2.03x10 5 e −3800 / T

3420e

−2650/ T

5

4 .46 x10 e

− 4100 / T

0.156e −14340 / RT

0.160e

−14380 / RT

0.0400 e

Q30 [kJ/kg]

[s ] 1815 e −55500 / RT

3010e −1870/ T (90℃)

1815e −55500 / RT

340

3810e −2390 / T (90℃)

671

1815e −55500 / RT

640

3255e −2175 / T (

Formulation of the Heat Generation Rate of Low-Temperature

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