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Thermogravimetric analysis of pretreated Turkish lignites
Fuel 78 (1999) 1109–1116
Thermogravimetric analysis of pretreated Turkish lignites ¨ nal K. Ceylan*, H. Karaca, Y. O Department of Chemical Engineering, Ino¨nu¨ University, Malatya, Turkey Received 7 April 1998; accepted 8 January 1999
Abstract The kinetics of nonisothermal pyrolysis of raw, demineralized or oxidized Turkish lignites have been investigated by thermogravimetry, differential thermal analysis and differential scanning calorimetry. The analyses were carried out in an inert atmosphere or in air atmosphere. The weight loss data indicate that pyrolysis characteristics of the lignites and the prevailing kinetic mechanism vary depending on temperature. The weight loss rates show essentially two regimes, and the major weight loss occurs in the range of 3008C–6508C. The results for the low temperature region (T , 2008C) suggest that the weight loss may be represented by 1.2–1.4-th order of reaction with an activation energy of approximately 15–35 kJ mol 21. The differential thermal analysis and the differential scanning calorimetry data gave similar values for the overall reaction order and activation energies for this temperature region. At the higher temperature region (T . 3008C), 1.2–1.8-th order of reaction shows a good fit with the weight loss data. The values of the estimated activation energies for this region vary from approximately 40 to 85 kJ mol 21 depending on the type of sample. However, the differential thermal analysis data suggest that the overall reaction order is around 1.0–1.2 and the effective activation energies vary approximately from 170 to 250 kJ mol 21. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lignite; Thermogravimetry; Kinetics
1. Introduction Coal pyrolysis is a very complex process and involves numerous reactions. Thermogravimetric analysis (TGA) provides a rapid quantitative method to examine the overall pyrolysis process, especially under nonisothermal conditions, and enables one to estimate the effective kinetic parameters for the overall decomposition reactions. Therefore, this technique has been widely used in recent years for investigation of pyrolysis, combustion and structural characteristics of fossil fuels such as coals, oil shales, tar sands [1–8]. Solid, liquid, and gaseous products are formed during the pyrolysis reactions of coal and some of these reactions are accompanied by appreciable weight and/or heat and temperature changes. The solid or some of the liquid products are trapped in coal matrix. Such reactions do not affect the weight loss characteristics, therefore, could not be registered with TGA. Depending on the pyrolysis conditions, some reactions may be endothermic and some other may be exothermic. It seems to be more appropriate to use
* Corresponding author. Fax: 0422-3410046. E-mail address: [email protected] (K. Ceylan)
differential thermal analysis (DTA) or differential scanning calorimetry (DSC) to follow such reactions. Several methods are available for the evaluation of TGA data for kinetic purposes, but, generally two main approaches have been used [8–13]. In the first, a single first-order reaction assumption was used for the representation of pyrolysis. In the second, the multiple parallel first-order reactions or the combined parallel and consecutive reactions model with a statistical distribution of activation energies was used for the description of the pyrolysis process [8,11,14–17]. This approach was later represented by nth order reaction model for the overall process and widely used to investigate coal pyrolysis. In this paper, TGA, DTA and DSC data from four Turkish lignites (raw, oxidized or demineralized) have been investigated and a preliminary global kinetic analysis has been carried out. The aim was to obtain some basic information on the nonisothermal pyrolysis of the raw or pretreated lignites and to discuss the effects of the treatments on the thermal decomposition characteristics. Basically, TGA and DTA data have been used for the estimation of the effective kinetic parameters of the process. The DSC analyses were used for a brief discussion of the combustion characteristics of the lignites.
0016-2361/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00009-5
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K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Table 1 Proximate analysis and sulfur distribution of raw or treated lignites (wt.% on air dried basis) a Lignite sample
Moisture
Ash
V.M.
F.C
ST
Sp
Ss
S0
Raw Kangal Oxidized Kangal Dem. Kangal
6.0 1.0 1.1
22.6 24.3 0.6
45.9 44.0 56.6
25.5 30.7 41.7
5.24 6.26 6.70
0.40 0.16 0.06
0.55 0.96 0.18
4.29 5.14 6.46
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
12.5 1.3 2.5
32.7 39.9 0.6
34.9 35.7 55.2
19.9 23.1 41.3
1.76 2.21 3.16
0.02 0.01 0.02
0.09 0.28 0.17
1.65 1.92 2.97
Raw Beypazari Oxidized Beypazari Dem. Beypazari
15.0 1.8 0.5
28.3 35.5 4.1
29.9 32.8 47.0
26.8 29.9 48.4
3.84 5.01 6.36
1.83 2.05 3.68
0.22 0.56 0.21
1.79 2.40 2.47
Raw Tuncbilek
11.1
35.2
27.7
26.0
0.99
0.70
0.08
0.21
a
V.M.: volatile matter, F.C.: fixed carbon, ST: total sulphur, Sp: pyritic sulphur, Ss: sulphatic sulphur, S0: organic sulphur, Dem.: demineralized.
2. Experimental 2.1. Sample preparation Approximately a sample of 10 kg in lumps from each of the four Turkish lignites has been grounded and sieved to pass 0.250 mm sieve. The samples were dried for 3 h in the laboratory atmosphere and then stored in plastic containers. These samples were used for the preparation of oxidized or demineralized lignite samples. For the preparation of the oxidized samples, the method described by Ndaji and Thomas et al. [18] was followed. A definite amount of a lignite (10 g) was placed into an oven at constant temperature of 2008C. Air was passed into the oven continuously at a constant rate of approximately 5 l min 21. After a 24 h of oxidation period, the sample was taken out of the oven, cooled to room temperature.
For the preparation of the demineralized samples, the method described by Samaras et al. [19] was followed. A definite amount of the lignites (10 g) was treated with first 5 N HCl (100 ml) by vigorous stirring in a flask under atmospheric nitrogen blanket at 608C for 1 h. The solid residue was recovered by filtration and washed with distilled water. The sample was then treated with concentrated HF (100 ml) in a teflon flask in the same manner. After the filtration, the solid residue was treated with concentrated HCl (100 ml), filtered, thoroughly washed with distilled water and then dried at 808C under vacuum. 2.2. The analyses The raw and the treated lignites were analyzed for proximate analysis and for the contents of sulfur forms by following the standard procedures [20,21]. The results for the proximate analysis and the sulfur forms are presented in Table 1. These samples were also subjected TGA, DTA and DSC analysis. The thermal analyses were carried out with the powdered samples with a size of approximately 10 mg and with a heating rate of 208C min 21. TGA and DTA were carried out in nitrogen atmosphere from room temperature to 9008C, but DSC analyses were carried out in air atmosphere from room temperature to 5008C. In the study of DTG and DSC, a -alumina was used as the reference material. Some analyses were repeated under identical conditions to see the reproducibility of the results and, some TGA were carried out at a heating rate of 408C min 21 to see the effect of the heating rate. A Shimadzu DT-50 model instrument was used in the analyses. 3. Results and discussion
Fig. 1. TGA curves of the selected lignite samples in nitrogen atmosphere (heating rate 20 oCmin -1). a: raw Kangal, b: oxidized Kangal, c: demineralized Kangal, d: oxidized Go¨lbasi, e: demineralized Go¨lbasi, f: raw Tuncbilek.
3.1. Thermogravimetric analysis The weight loss traces of the selected raw or treated lignite samples, for a heating rate of 208C min 21 in inert
K. Ceylan et al. / Fuel 78 (1999) 1109–1116 Table 2 The maximum weight loss temperatures, the ignition temperatures and the maximum heat flow temperatures a Lignite sample
T1 (8C)
T2 (8C)
Ti (8C)
Tm (8C)
e (kJ g 21)
Raw Kangal Oxidized Kangal Dem. Kangal
73.8 75.6 71.8
443 479 523
205 200 188
312 341 390
0.230 0.195 0.244
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
79.8 80.1 74.0
447 477 488
220 219 205
370 373 358
0.221 0.163 0.327
Raw Beypazari Oxidized Beypazari Dem. Beypazari
76.8 81.3 86.6
481 492 516
225 — —
384 — —
0.231 — —
Raw Tuncbilek
71
476
221
381
0.242
a
T1, T2: the maximum weight loss rate temperatures for the low or high temperature regions, respectively; Ti: ignition temperature, Tm: maximum heat flow temperature, e energy flow.
atmosphere are represented in Fig. 1. The figure indicates that all the samples lose almost completely their moisture water below 1508C. The essential weight loss occurs mainly in the 3008C–6508C region. The characteristic temperatures are given in Table 2. It is seen from the table that the maximum devolatilization temperatures are about 758C–858C for the lower temperature region but about 4458C–5208C for the higher temperature region. Fig. 1 suggests that the thermal decomposition of all the lignites start around 2508C. A slow weight loss was observed for some samples in the temperature range of 2508C–3008C. This loss may be attributed to the loss of a small amount of pyrolysis water as a
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result of the decomposition of the phenolic structures, the carbonyl groups or the peroxy radicals [15,22]. The rapid devolatilization after 3008C is associated with the primary carbonization. At the temperatures above 6508C, a weight loss of 5%–10% is observed, which may be related to the secondary pyrolysis. The weight loss almost ceases after 8008C and the heating rate has almost no effect on the weight loss after this temperature. Similar results have been reported by the other researchers [8–11]. The effects of a higher heating rate are similar to those reported in the literature. The temperatures of maximum rate of weight loss in the higher temperature region may be used as an indicator for the coal reactivity in pyrolysis or devolatilization [10]. As the reactivity increases, the maximum devolatilization temperature is expected to decrease. In this respect, the data in Table 2 suggest that the raw lignites are more reactive than their oxidized or demineralized forms. In other words, the reactivity order of a certain lignite and its derivatives for the devolatilization may be stated as: Raw lignite . Oxidized lignite . Demineralized lignite. Similarly, the reactivity order for the raw lignites is: Kangal lignite Go¨lbasi lignite . Beypazari lignite Tuncbilek lignite. It is known that Kangal and Go¨lbasi lignites are younger than the other two. Therefore, this order of reactivity correlates approximately with rank. However, the reactivity differences seem to be not great as the maximum devolatilization temperatures are not so much different.
3.2. Differential thermal analysis
Fig. 2. DTA curves of the lignite samples in nitrogen atmosphere (heating rate 20 oCmin -1) a: raw Beypazari, b: demineralized Beypazari, c: oxidized Beypazari, d: raw Kangal, e: demineralized Kangal, f: oxidized Kangal, g: raw Go¨lbasi, i: oxidized Go¨lbasi, h: demineralized Go¨lbasi, j: raw Tuncbilek
DTA enables to trace the temperature changes during the reactions. The DTA traces (normalized to unit weight) of the samples are represented in Fig. 2. The figure indicates that all the samples show at least two endothermic peaks, but some samples show three peaks. The first endotherm exhibited below 1508C is attributed to the evaporation of moisture water. The broad, shallow endotherms, exhibited by some samples (generally by the oxidized lignites) in the range of 2008C–3008C may be attributed to the loss of a small amount of pyrolysis water, as mentioned earlier. The DTA curves also suggest that the primary carbonization starts around 3508C, but main degradation is in the range of 4008C–5008C. A third endothermic peak was observed in some samples after 6008C. As the weight loss is negligible beyond this temperature, and as some of the demineralized lignites also show this peak, it may essentially be attributed to the secondary pyrolysis [8,15] i.e. to the coking of the heavy fractions of the degradation products to produce a carbonaceous residue. Elder and Harris [15] noted that pyrite/pyrrhotite transformation also takes place around this temperature. However, the contribution of this
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These observations suggest that the reactivity order for ignition of a certain lignite and its pretreated forms be: Demineralized lignite . Oxidized lignite Raw lignite.
Fig. 3. DSC curves of the selected lignite samples in air atmosphere (heating rate 20 oCmin -1) a: raw Kangal, b: oxidized Kangal, c: demineralized Kangal, d: raw Go¨lbasi, e: demineralized Go¨lbasi, f: oxidized Go¨lbasi.
transformation to this peak is expected to be minor as the lignites are relatively low pyritic sulphur content.
3.3. Differential scanning calorimetry analysis DSC enables one to follow the heat changes during the reactions. Some representative thermograms for the raw or the oxidized lignites heated in air atmosphere are demonstrated in Fig. 3. It is seen from the figure that the DSC traces show essentially one endothermic peak and one exothermic peak. The endothermic peaks represent the evaporation of moisture water and the exothermic peaks represent the lignite–oxygen reactions (or the combustion) of the samples. As the analyses have been carried out in air atmosphere, the onset of an exothermic peak may be assumed as the ignition temperature and these peaks supply information about the combustion characteristics of the samples. The overall effect after this temperature is exothermic with a maximum heat-flow in the range of 3208C– 3908C. From the enthalpy measurements, an enthalpy value of 0.20–0.33 kJ g 21 was obtained depending on the type of the samples. The ignition temperatures (Ti), the maximum heat flow temperatures (Tm) and the measured heat flows (e ) are also given in Table 2. A lower ignition temperature may be as a result of a higher reactivity for the lignite–oxygen reactions [10]. Based on this fact, the ignition temperatures may be used for a comparison of reactivity for the combustion reactions. The data in Table 2 indicate that the ignition temperatures of the demineralized lignites are lower approximately 158C– 208C than those of the raw lignites or the oxidized lignites.
The increase in the reactivity caused by demineralization may be a consequence of the changes in the physical structure of the lignites during the treatment [16]. After the demineralization processes, porosity, therefore the total surface area is, probably, increased as almost all of the inorganic part of the lignites is removed. These structural changes may result in an increase in the concentration of the active surface for the reaction of carbon–oxygen. Consequently, the reaction starts at lower temperatures. The ignition temperatures may also be used for a comparison of the reactivities of the raw lignites. The results indicate that the order of reactivity for the lignite–oxygen reactions is almost the same with the order of devolatilization, as given before. Similar to the ignition temperatures, the maximum heat flow temperatures may also be used for comparison of reactivity of the samples. Coal combustion is a heterogeneous reaction and the rate of heat flow is a function of the combustion rate. A lower temperature for the maximum heat flow rate is a consequence of a higher combustion rate. Based on this fact, the temperatures of the maximum heat flow given in Table 2 may be used for a definition of a new reactivity order. A comparison of these temperatures for the raw lignites and its pretreated forms gives different reactivity pattern depending on the types of lignites. For example, raw Kangal lignite seems to be more reactive than its oxidized or demineralized forms. In contrast, raw Go¨lbasi lignite seems to be less reactive than its demineralized form. These results suggest that the effect of the treatments on the rate of heat flows, or on the rates of the carbon–oxygen reactions are not the same for different lignites. Structural changes in solid phase during the treatments, as mentioned earlier, may be an important parameter in this different behavior although several other parameters may also affect the rate of the reactions [2,4,5,23,24]. Ismail and Walker[24] reported that the geometric configuration of the char surface or the nature of C-bonding on the surface is effective in the char–oxygen reactions. Based on the earlier discussion and on the experimental data, it may be concluded that the effects of the treatments on the solid phase structure of the raw lignites are different. Therefore, the reactivity pattern for carbon–oxygen reactions, consequently the heat flow rates, are changed in a different manner for the lignites. Nevertheless, a comparison of the temperatures of maximum heat flow for the raw lignites gives almost the same reactivity pattern with that for the reactivity in the devolatilization. This is an expected result because the devolatilization is the fist step in combustion. 3.4. A kinetic approach The kinetic analysis of nonisothermal pyrolysis, in
K. Ceylan et al. / Fuel 78 (1999) 1109–1116
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values [15,16]. Inserting this definition into Eq. (4) gives da=dt A 2E 6 exp 1 2 an q RT The order of reaction n can be determined by taking logarithm of this equation and then plotting ln {(da /dT)/ (1 2 a ) n} versus 1/T. A rectilinear plot may be obtained with a correct value for n. Coats and Redfern [25] have integrated Eq. (6) by expanding it into series with the boundary conditions of a 0 for T T0 and a a for T T. For the case of n ± 1, they obtained the following equation by ignoring the higher order terms of the series, 1 2 1 2 a 12n AR 2RT 2E 12 exp 7 qE E RT 1 2 nT 2 Eq. (7) may be further simplified if it is assumed 2RT p E. Based on these assumptions, the following equation may be used for estimating the kinetic parameters from TGA data: Fig. 4. Determination of the kinetic parameters by using TGA data for the high temperature region. W: raw Tuncbilek lignite, L: raw Beypazari lignite, O: oxidized Go¨lbasi lignite, X: demineralized Kangal lignite, K: raw Go¨lbasi lignite, (the n values for each sample are given in Table 4).
ln
ln
k Aexp
2E RT
T T0 1 qt
1 2 3
where a is the fraction decomposed, f(a ) a function of the degree of the reaction, t the reaction time, k the rate constant, T absolute temperature, E and A are the activation energy and pre-exponential factor, respectively, R is the gas constant, q the heating rate and T0 the starting temperature. By combining Eqs. 1–3, the decomposition rate may be expressed as: da A 2E f a exp 4 dt q RT In the analysis of TGA data, several definitions have been used for the fractional weight loss, a , but in this work, a is defined as in the same way with Elder and Harris [15]:
a
1 2 ft 1 2 f∞
5
where ft and f∞ are the instantaneous and the final values of the weight fractions, respectively. A graph of a versus temperature gives a modified TGA plot which can be used for kinetic analysis. Based on recent calculations, f(a ) is defined as f(a ) (1 2 a ) n (n is overall reaction order) with singular E and A
8
With the same assumptions as given before, the integration of Eq. (6) for the first order reaction (n 1) gives:
general, is based on the following three equations [9]: da kf a dt
1 2 1 2 a 12n AR E 2 ln 2 qE RT 1 2 nT
2ln 1 2 a AR E 2 ln qE RT T2
9
By using the a values, a plot of left side of Eqs. (8) or (9) versus 1/T should give straight lines for a correct value of n. The conventional Arrhenius parameters, A and E, can then be estimated from the intercept and the slope of these lines. However, a straight line with a high correlation coefficient could not obtained in this study, when the data of whole temperature range were processed together, even if the weight changes below 1008C (this amounted to approximately 1%–10% of the total weight change) has been neglected in the evaluation of a values. The shape of TGA traces in Fig. 1 suggest that the weight loss rates and the prevailing reaction mechanism change depending on the temperature regions. Instead of the estimation of the overall kinetic parameters for the whole temperature range, therefore, it seems be more convenient to apply Eqs. (8) or (9) for definite temperature intervals. Accordingly, the effective kinetic parameters, in this work, are estimated for two separate temperature regions. The first is the lower temperature region (T , 2008C) where mainly the moisture water is removed and the second is the higher temperature region (T . 3008C) where generally 80% of the conversion is took place. The two-stage kinetics is observed also by many other workers [8,16,17]. In the evaluation of Eq. (8), the best value for n was searched by a computer program in the range from 0.2 to 2.0 to get a straight line. The n value giving a straight line with a highest correlation coefficient was assumed to be the order of the overall pyrolysis reactions. The values of E and A were then calculated from the slope and the intercept of the line, respectively. A regression
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Table 3 The pyrolysis kinetic parameters estimated from TGA data (heating rate 208C min 21) a Low temp. (T , 2008C) region
High temp. (T . 3008C) region
Lignite sample
E
A
n
R
E
A
n
R
Raw Kangal Oxidized Kangal Dem. Kangal
34.5 19.2 19.8
0.37 × 10 4 1.15 × 10 6 1.21 × 10 4
1.4 1.6 1.6
0.995 0.975 0.995
71.5 57.5 38.5
0.56 × 10 4 1.54 × 10 5 1.54 × 10 6
1.6 1.8 1.8
0.993 0.983 0.980
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
16.8 15.6 21.2
0.91 × 10 6 0.77 × 10 6 1.23 × 10 6
1.4 1.6 1.6
0.985 0.980 0.990
47.5 57.5 49.6
2.45 × 10 5 7.70 × 10 5 2.75 × 10 5
1.8 1.8 1.8
0.993 0.983 0.990
Raw Beypazari Oxidized Beypazari Dem. Beypazari
32.9 22.3 15.2
0.87 × 10 4 0.29 × 10 4 5.45 × 10 4
1.2 1.6 1.4
0.990 0.980 0.975
85.3 47.5 47.5
5.67 × 10 4 8.53 × 10 5 1.92 × 10 4
1.8 1.8 1.8
0.989 0.980 0.982
Raw Tuncbilek
39.2
0.28 × 10 4
1.4
0.992
88.9
3.13 × 10 5
1.6
0.990
a
21
21
E: energy of activation (kJ mol ), A: pre-exponential factor for Arrhenius statement (s ), n: overall reaction order, R: correlation coefficient.
analysis with “the least squares method” was employed in the estimation of n, E and A for each temperature region. Some representative plots of Eq. (8) for the high temperature region are presented in Fig. 4 for the selected samples. Similar plots are obtained also for the other samples, for both low and high temperature regions. The n values and resulting global kinetic parameters (i.e. A and E values) calculated from these plots are summarized in Table 3. Based on the results of the analyses with a heating rate of 408C min 21, it was concluded that the heating rates have relatively small effect on the kinetic parameters. The data in Table 3 indicate that the weight losses may generally be represented by Eq. (8) with the n values in the range 1.2–1.8, depending on the type of sample. The data in the literature also indicate that the overall reaction order for the nonisothermal decomposition of coals varies from 1.0 to 2.5. For example, Skala et al. [8] reported that TGA data of an oil shale fit the reaction order of 1.0, but, Elder and Harris [15] reported that TGA data of Kentucky coals fit the reaction order of 1.5–2.5. Similarly, the reported activation energies vary in a wide range depending on the type of sample. It seen from the table that the activation energies for the low temperature region are relatively low. This result suggests that the devolatilization process in this region is essentially diffusion controlled. This is an expected result as the weight loses in this region essentially originates from the removal of moisture water. The energies for the oxidized or demineralized samples are generally lower than for the parent raw lignites. As the treated lignites are low moisture content, this result implies that the energies may depend on the initial moisture content. However, the structural changes in solid phase, as discussed earlier, may also affect the activation energies if the process is diffusion controlled. For the higher temperature region, the activation energies are appreciably higher and this suggests that the devolatilization be, essentially, chemically controlled. The estimated values for E and A change appreciably depending on the types of the
samples. The data given in the literature for the high temperature region indicate that the estimated energy values change in the range of 50–200 kJ mol 21, depending on the type of sample [11,14,15,17]. In this respect, the values reported here are in accord with those given in the literature. For example, Ko¨k and Okandan [26] reported activation energies of 60–80 kJ mol 21 for some Turkish lignites. Haddadin and Tawarah [16], reported activation energies of 16–20 kJ mol 21 for the low temperature range, but 50– 80 kJ mol 21 for the high temperature region for the nonisothermal pyrolysis of Jordan oil shale. Elder and Harris [15] reported higher activation energies for bituminous coals. Similarly, the reported pre-exponential factors also vary in the range of 10 3 to 10 14 s 21. The data in Table 3 also indicate that either oxidized or demineralized lignites exhibit appreciably lower activation energies in comparison to those of parent lignites. This result suggest that both oxidation and demineralization processes increase the reactivity of the lignites for the devolatilization. Kudynska and Buckmaster [22] suggested that some peroxides are formed during oxidation and –O–O– bonds in these structures easily undergo unimolecular homolysis even at low temperatures. Therefore, the primary carbonization of the oxidized samples starts at relatively lower temperatures and requires lower energy of activation. Similarly, the acid treatments during the demineralization process cause the cleavage several types of bonding [27]. These type effects may result in lower activation energies for the devolatilization process. In fact, the energies estimated from the thermogravimetric analyses are essentially the effective energies for the sum of different reactions that occur simultaneously. The real activation energies for several reactions may be much higher. DTA or DSC data also enable to estimate the effective kinetic parameters [7–9,16]. Haddadin and Tawarah [16] used directly the differential form of the nth order rate model (Eq. (6)) to estimate the kinetic parameters for each
K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Fig. 5. Plots of nth order equation for the first endotherm of DSC curve of raw Kangal lignite.
peak region in the DTA or DSC curves. A straight line should be obtained by plotting ln{da /dT/(1 2 a ) n} versus 1/T if the order n is known for the peak regions. The correct n value, in this work, was searched by a computer program as mentioned earlier. A relatively different definition of the conversion fraction, a (as is demonstrated in Fig. 3), has been used in the evaluation of DSC and DTA data. In this definition, the value of a ranges from 0.0 to 1.0 at each peak region. The plots of nth order rate model for the first endotherm in DSC curve of raw Kangal lignite and for the second endotherm in DTA curve of demineralized Kangal lignite are given in Figs. 5 and 6, respectively. Similar plots were obtained from the peaks in the DSC and DTA traces of the all samples. The results indicated that generally firstorder reaction model gives straight lines with high correlation coefficients. The kinetic parameters A and E are then estimated from the intercept and the slope of these lines. The estimated activation energies by this method for the pyrolysis (from DTA data) and for the combustion (from DSC data) are summarized in Table 4. A comparison of the data given in Tables 3 and 4
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indicates that the activation energies estimated from TGA, DSC and DTA for the low temperature region are in a good agreement. These results indicate that the gaseous atmosphere has almost no effects on the evaporation of moisture water in this temperature region. However, for the higher temperature region, the type of gaseous atmosphere is an effective parameter. The DSC derived energy values in Table 4 are for the combustion reactions and vary approximately from 50 to 60 kJ mol 21. The TGA or DTA derived values are for the pyrolysis reactions in inert atmosphere. Although the TGA derived energy values vary from approximately 40 to 85 kJ mol 21, the DTA derived values vary from approximately 170 to 260 kJ mol 21. The results indicate that the activation energies estimated from the different methods (i.e. TGA and DTA) are significantly different and DTA derived energy values 2–3 times higher than those derived from TGA. Similar results are reported also by other researchers [8]. This situation essentially originates from the differences in basic data obtained by these methods. In TGA, the weight losses; but in DTA, the temperature changes are registered versus temperature or time. For a simple physical or chemical process, the data obtained from the different methods can easily be correlated. However, for the case of coal pyrolysis, the correlations between the evolution of volatiles and the temperature changes or the heat absorptions are very complicated. The aforementioned results imply that a substantial portion of the pyrolysis reactions at higher temperatures is accompanied by temperature changes. Some of these reactions are also accompanied by weight loss, but some are not. The reactions that produce volatile products (liquids or gases) seem to have lower activation energies in comparison to those reactions that do not produce volatile products.
4. Conclusions The thermogravimetric data discussed earlier suggest that
Fig. 6. Plots of nth order equation for second endotherm of DTA curve of demineralized Kangal lignite.
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K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Table 4 The first order kinetic parameters estimated from DSC data (in air atmosphere) and DTA data (in nitrogen atmosphere) (heating rate 208C min 21) a DSC data
DTA data
Endothermic peak exothermic peak second endotherm
Exothermic peak
Second endotherm
Lignite sample
E
A
E
A
E
A
Raw Kangal Oxidized Kangal Dem. Kangal
28.6 23.9 23.6
1.00 × 10 3 1.12 × 10 3 1.21 × 10 3
52.6 55.3 55.6
2.83 × 10 3 4.21 × 10 3 2.30 × 10 3
198.6 230.0 225.4
1.38 × 10 12 3.73 × 10 12 2.67 × 10 12
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
19.6 21.6 21.8
0.14 × 10 3 0.52 × 10 3 1.04 × 10 3
51.6 60.3 50.2
1.56 × 10 3 8.18 × 10 3 1.26 × 10 3
236.1 226.5 177.3
5.08 × 10 13 12.60 × 10 13 3.97 × 10 10
Raw Beypazari Oxidized Beypazari Dem. Beypazari
28.1 — —
0.61 × 10 3 — —
56.3 — —
5.10 × 10 3 — —
258.6 189.8 256.2
1.14 × 10 14 8.87 × 10 12 1.85 × 10 15
Raw Tuncbilek
29.1
0.85 × 10 4
56.4
5.41 × 10 4
168.4
0.47 × 10 11
a
See Tables 1 and 3 for the nomenclature.
the pyrolysis process of the lignites involve at least two kinetically active steps. The major devolatilization starts after 3008C–3508C and the order of overall reaction changes depending on the type of sample and temperature region. The weight loss process is essentially diffusion controlled at the lower temperature region, but is chemically controlled at the higher temperature region. The effective activation energies estimated by different methods for the lower temperature region are similar but the energies for the higher temperature region are appreciably different. The DTA estimated energies are 2–3 times higher than the energies estimated from TGA. This result implies that the most of the higher temperature reactions that do not result in appreciable weight loss proceed with higher activation energies. The activation energies, or the peak temperatures for devolatilization or for ignition of the lignites suggest that the pretreatments such as oxidation or demineralization generally increase the reactivity for devolatilization or for combustion. This result implies that the pretreatments appreciably affect the solid phase structure.
Acknowledgements It is a pleasure to acknowledge the support of this work ¨ .A.F.96/14) by the Research Foundation of (Project No: I.U Ino¨nu¨ University.
References [1] Ottaway M. Fuel 1982;61:713–716. [2] Ghetti P, De Robertis U, D’Antone S, Villani M, Chiellini E. Fuel 1985;64:950–955. [3] De Koranyi A. Thermochimica Acta 1987;110:527–533. [4] Stanmore BR. Fuel 1991;70:1485–1487. [5] Guillena MJM, Solano AL, de Lecea SM. Fuel 1992;71:579–583. [6] Thring RW, Chornet E, Overend RP. Fuel 1993;72:1355–1356. [7] Mateos JMJ, Quintero LC, Rial C. Fuel 1996;75:1691–1700. [8] Skala D, Kopsch H, Sokic M, Neumann H-J, Jovanovic J. Fuel 1987;66:1185–1191. [9] Keattch CJ, Dollimore D. An introduction to thermogravimetry. 2nd. London: Heyden & Sons Ltd, 1975. [10] Ciuryla VT, Weimer RF, Bivans DA, Motika SA. Fuel 1979;58:748–754. [11] McCown MS, Harrison DP. Fuel 1982;61:1149–1154. [12] Yinnon H, Uhlmann DR. J. Non-Crystalline Solids 1983;54:253–275. [13] Bansal NP, Bruce AJ, Doremus RH, Moynihan CT. J. Non-Crystalline Solids 1985;70:379–396. [14] Campbell JH, Koskinas GH, Stout ND. Fuel 1978;57:372–376. [15] Elder JP, Harris MB. Fuel 1984;63:262–267. [16] Haddadin RA, Tawarah KM. Fuel 1980;59:539–543. [17] Williams PFV. Fuel 1985;64:540–545. [18] Ndaji FE. Thomas K.M. Fuel 1995;74:932–937. [19] Samaras P, Diamadopoulos E, Sakellaropoulos GP. Carbon 1991;29:1181–1190. [20] Turkish Standards, 1966, TS 329. [21] British Standards, 1977, BS 1016, Part VI. [22] Kudynska J, Buckmaster HA. Fuel 1996;75:872–878. [23] Morgan PA, Robertson SD, Unsworth JF. Fuel 1987;66:210–215. [24] Ismail IMK, Walker PL. Fuel 1989;68:1456–1460. [25] Coats AV, Redfern JP. Nature 1964;201:68. [26] Ko¨k MV, Okandan E. Fuel 1994;73:500–504. ¨ nal Y, Ceylan K. Fuel 1995;74:972–977. [27] O
Thermogravimetric analysis of pretreated Turkish lignites ¨ nal K. Ceylan*, H. Karaca, Y. O Department of Chemical Engineering, Ino¨nu¨ University, Malatya, Turkey Received 7 April 1998; accepted 8 January 1999
Abstract The kinetics of nonisothermal pyrolysis of raw, demineralized or oxidized Turkish lignites have been investigated by thermogravimetry, differential thermal analysis and differential scanning calorimetry. The analyses were carried out in an inert atmosphere or in air atmosphere. The weight loss data indicate that pyrolysis characteristics of the lignites and the prevailing kinetic mechanism vary depending on temperature. The weight loss rates show essentially two regimes, and the major weight loss occurs in the range of 3008C–6508C. The results for the low temperature region (T , 2008C) suggest that the weight loss may be represented by 1.2–1.4-th order of reaction with an activation energy of approximately 15–35 kJ mol 21. The differential thermal analysis and the differential scanning calorimetry data gave similar values for the overall reaction order and activation energies for this temperature region. At the higher temperature region (T . 3008C), 1.2–1.8-th order of reaction shows a good fit with the weight loss data. The values of the estimated activation energies for this region vary from approximately 40 to 85 kJ mol 21 depending on the type of sample. However, the differential thermal analysis data suggest that the overall reaction order is around 1.0–1.2 and the effective activation energies vary approximately from 170 to 250 kJ mol 21. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Lignite; Thermogravimetry; Kinetics
1. Introduction Coal pyrolysis is a very complex process and involves numerous reactions. Thermogravimetric analysis (TGA) provides a rapid quantitative method to examine the overall pyrolysis process, especially under nonisothermal conditions, and enables one to estimate the effective kinetic parameters for the overall decomposition reactions. Therefore, this technique has been widely used in recent years for investigation of pyrolysis, combustion and structural characteristics of fossil fuels such as coals, oil shales, tar sands [1–8]. Solid, liquid, and gaseous products are formed during the pyrolysis reactions of coal and some of these reactions are accompanied by appreciable weight and/or heat and temperature changes. The solid or some of the liquid products are trapped in coal matrix. Such reactions do not affect the weight loss characteristics, therefore, could not be registered with TGA. Depending on the pyrolysis conditions, some reactions may be endothermic and some other may be exothermic. It seems to be more appropriate to use
* Corresponding author. Fax: 0422-3410046. E-mail address: [email protected] (K. Ceylan)
differential thermal analysis (DTA) or differential scanning calorimetry (DSC) to follow such reactions. Several methods are available for the evaluation of TGA data for kinetic purposes, but, generally two main approaches have been used [8–13]. In the first, a single first-order reaction assumption was used for the representation of pyrolysis. In the second, the multiple parallel first-order reactions or the combined parallel and consecutive reactions model with a statistical distribution of activation energies was used for the description of the pyrolysis process [8,11,14–17]. This approach was later represented by nth order reaction model for the overall process and widely used to investigate coal pyrolysis. In this paper, TGA, DTA and DSC data from four Turkish lignites (raw, oxidized or demineralized) have been investigated and a preliminary global kinetic analysis has been carried out. The aim was to obtain some basic information on the nonisothermal pyrolysis of the raw or pretreated lignites and to discuss the effects of the treatments on the thermal decomposition characteristics. Basically, TGA and DTA data have been used for the estimation of the effective kinetic parameters of the process. The DSC analyses were used for a brief discussion of the combustion characteristics of the lignites.
0016-2361/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00009-5
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Table 1 Proximate analysis and sulfur distribution of raw or treated lignites (wt.% on air dried basis) a Lignite sample
Moisture
Ash
V.M.
F.C
ST
Sp
Ss
S0
Raw Kangal Oxidized Kangal Dem. Kangal
6.0 1.0 1.1
22.6 24.3 0.6
45.9 44.0 56.6
25.5 30.7 41.7
5.24 6.26 6.70
0.40 0.16 0.06
0.55 0.96 0.18
4.29 5.14 6.46
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
12.5 1.3 2.5
32.7 39.9 0.6
34.9 35.7 55.2
19.9 23.1 41.3
1.76 2.21 3.16
0.02 0.01 0.02
0.09 0.28 0.17
1.65 1.92 2.97
Raw Beypazari Oxidized Beypazari Dem. Beypazari
15.0 1.8 0.5
28.3 35.5 4.1
29.9 32.8 47.0
26.8 29.9 48.4
3.84 5.01 6.36
1.83 2.05 3.68
0.22 0.56 0.21
1.79 2.40 2.47
Raw Tuncbilek
11.1
35.2
27.7
26.0
0.99
0.70
0.08
0.21
a
V.M.: volatile matter, F.C.: fixed carbon, ST: total sulphur, Sp: pyritic sulphur, Ss: sulphatic sulphur, S0: organic sulphur, Dem.: demineralized.
2. Experimental 2.1. Sample preparation Approximately a sample of 10 kg in lumps from each of the four Turkish lignites has been grounded and sieved to pass 0.250 mm sieve. The samples were dried for 3 h in the laboratory atmosphere and then stored in plastic containers. These samples were used for the preparation of oxidized or demineralized lignite samples. For the preparation of the oxidized samples, the method described by Ndaji and Thomas et al. [18] was followed. A definite amount of a lignite (10 g) was placed into an oven at constant temperature of 2008C. Air was passed into the oven continuously at a constant rate of approximately 5 l min 21. After a 24 h of oxidation period, the sample was taken out of the oven, cooled to room temperature.
For the preparation of the demineralized samples, the method described by Samaras et al. [19] was followed. A definite amount of the lignites (10 g) was treated with first 5 N HCl (100 ml) by vigorous stirring in a flask under atmospheric nitrogen blanket at 608C for 1 h. The solid residue was recovered by filtration and washed with distilled water. The sample was then treated with concentrated HF (100 ml) in a teflon flask in the same manner. After the filtration, the solid residue was treated with concentrated HCl (100 ml), filtered, thoroughly washed with distilled water and then dried at 808C under vacuum. 2.2. The analyses The raw and the treated lignites were analyzed for proximate analysis and for the contents of sulfur forms by following the standard procedures [20,21]. The results for the proximate analysis and the sulfur forms are presented in Table 1. These samples were also subjected TGA, DTA and DSC analysis. The thermal analyses were carried out with the powdered samples with a size of approximately 10 mg and with a heating rate of 208C min 21. TGA and DTA were carried out in nitrogen atmosphere from room temperature to 9008C, but DSC analyses were carried out in air atmosphere from room temperature to 5008C. In the study of DTG and DSC, a -alumina was used as the reference material. Some analyses were repeated under identical conditions to see the reproducibility of the results and, some TGA were carried out at a heating rate of 408C min 21 to see the effect of the heating rate. A Shimadzu DT-50 model instrument was used in the analyses. 3. Results and discussion
Fig. 1. TGA curves of the selected lignite samples in nitrogen atmosphere (heating rate 20 oCmin -1). a: raw Kangal, b: oxidized Kangal, c: demineralized Kangal, d: oxidized Go¨lbasi, e: demineralized Go¨lbasi, f: raw Tuncbilek.
3.1. Thermogravimetric analysis The weight loss traces of the selected raw or treated lignite samples, for a heating rate of 208C min 21 in inert
K. Ceylan et al. / Fuel 78 (1999) 1109–1116 Table 2 The maximum weight loss temperatures, the ignition temperatures and the maximum heat flow temperatures a Lignite sample
T1 (8C)
T2 (8C)
Ti (8C)
Tm (8C)
e (kJ g 21)
Raw Kangal Oxidized Kangal Dem. Kangal
73.8 75.6 71.8
443 479 523
205 200 188
312 341 390
0.230 0.195 0.244
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
79.8 80.1 74.0
447 477 488
220 219 205
370 373 358
0.221 0.163 0.327
Raw Beypazari Oxidized Beypazari Dem. Beypazari
76.8 81.3 86.6
481 492 516
225 — —
384 — —
0.231 — —
Raw Tuncbilek
71
476
221
381
0.242
a
T1, T2: the maximum weight loss rate temperatures for the low or high temperature regions, respectively; Ti: ignition temperature, Tm: maximum heat flow temperature, e energy flow.
atmosphere are represented in Fig. 1. The figure indicates that all the samples lose almost completely their moisture water below 1508C. The essential weight loss occurs mainly in the 3008C–6508C region. The characteristic temperatures are given in Table 2. It is seen from the table that the maximum devolatilization temperatures are about 758C–858C for the lower temperature region but about 4458C–5208C for the higher temperature region. Fig. 1 suggests that the thermal decomposition of all the lignites start around 2508C. A slow weight loss was observed for some samples in the temperature range of 2508C–3008C. This loss may be attributed to the loss of a small amount of pyrolysis water as a
1111
result of the decomposition of the phenolic structures, the carbonyl groups or the peroxy radicals [15,22]. The rapid devolatilization after 3008C is associated with the primary carbonization. At the temperatures above 6508C, a weight loss of 5%–10% is observed, which may be related to the secondary pyrolysis. The weight loss almost ceases after 8008C and the heating rate has almost no effect on the weight loss after this temperature. Similar results have been reported by the other researchers [8–11]. The effects of a higher heating rate are similar to those reported in the literature. The temperatures of maximum rate of weight loss in the higher temperature region may be used as an indicator for the coal reactivity in pyrolysis or devolatilization [10]. As the reactivity increases, the maximum devolatilization temperature is expected to decrease. In this respect, the data in Table 2 suggest that the raw lignites are more reactive than their oxidized or demineralized forms. In other words, the reactivity order of a certain lignite and its derivatives for the devolatilization may be stated as: Raw lignite . Oxidized lignite . Demineralized lignite. Similarly, the reactivity order for the raw lignites is: Kangal lignite Go¨lbasi lignite . Beypazari lignite Tuncbilek lignite. It is known that Kangal and Go¨lbasi lignites are younger than the other two. Therefore, this order of reactivity correlates approximately with rank. However, the reactivity differences seem to be not great as the maximum devolatilization temperatures are not so much different.
3.2. Differential thermal analysis
Fig. 2. DTA curves of the lignite samples in nitrogen atmosphere (heating rate 20 oCmin -1) a: raw Beypazari, b: demineralized Beypazari, c: oxidized Beypazari, d: raw Kangal, e: demineralized Kangal, f: oxidized Kangal, g: raw Go¨lbasi, i: oxidized Go¨lbasi, h: demineralized Go¨lbasi, j: raw Tuncbilek
DTA enables to trace the temperature changes during the reactions. The DTA traces (normalized to unit weight) of the samples are represented in Fig. 2. The figure indicates that all the samples show at least two endothermic peaks, but some samples show three peaks. The first endotherm exhibited below 1508C is attributed to the evaporation of moisture water. The broad, shallow endotherms, exhibited by some samples (generally by the oxidized lignites) in the range of 2008C–3008C may be attributed to the loss of a small amount of pyrolysis water, as mentioned earlier. The DTA curves also suggest that the primary carbonization starts around 3508C, but main degradation is in the range of 4008C–5008C. A third endothermic peak was observed in some samples after 6008C. As the weight loss is negligible beyond this temperature, and as some of the demineralized lignites also show this peak, it may essentially be attributed to the secondary pyrolysis [8,15] i.e. to the coking of the heavy fractions of the degradation products to produce a carbonaceous residue. Elder and Harris [15] noted that pyrite/pyrrhotite transformation also takes place around this temperature. However, the contribution of this
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These observations suggest that the reactivity order for ignition of a certain lignite and its pretreated forms be: Demineralized lignite . Oxidized lignite Raw lignite.
Fig. 3. DSC curves of the selected lignite samples in air atmosphere (heating rate 20 oCmin -1) a: raw Kangal, b: oxidized Kangal, c: demineralized Kangal, d: raw Go¨lbasi, e: demineralized Go¨lbasi, f: oxidized Go¨lbasi.
transformation to this peak is expected to be minor as the lignites are relatively low pyritic sulphur content.
3.3. Differential scanning calorimetry analysis DSC enables one to follow the heat changes during the reactions. Some representative thermograms for the raw or the oxidized lignites heated in air atmosphere are demonstrated in Fig. 3. It is seen from the figure that the DSC traces show essentially one endothermic peak and one exothermic peak. The endothermic peaks represent the evaporation of moisture water and the exothermic peaks represent the lignite–oxygen reactions (or the combustion) of the samples. As the analyses have been carried out in air atmosphere, the onset of an exothermic peak may be assumed as the ignition temperature and these peaks supply information about the combustion characteristics of the samples. The overall effect after this temperature is exothermic with a maximum heat-flow in the range of 3208C– 3908C. From the enthalpy measurements, an enthalpy value of 0.20–0.33 kJ g 21 was obtained depending on the type of the samples. The ignition temperatures (Ti), the maximum heat flow temperatures (Tm) and the measured heat flows (e ) are also given in Table 2. A lower ignition temperature may be as a result of a higher reactivity for the lignite–oxygen reactions [10]. Based on this fact, the ignition temperatures may be used for a comparison of reactivity for the combustion reactions. The data in Table 2 indicate that the ignition temperatures of the demineralized lignites are lower approximately 158C– 208C than those of the raw lignites or the oxidized lignites.
The increase in the reactivity caused by demineralization may be a consequence of the changes in the physical structure of the lignites during the treatment [16]. After the demineralization processes, porosity, therefore the total surface area is, probably, increased as almost all of the inorganic part of the lignites is removed. These structural changes may result in an increase in the concentration of the active surface for the reaction of carbon–oxygen. Consequently, the reaction starts at lower temperatures. The ignition temperatures may also be used for a comparison of the reactivities of the raw lignites. The results indicate that the order of reactivity for the lignite–oxygen reactions is almost the same with the order of devolatilization, as given before. Similar to the ignition temperatures, the maximum heat flow temperatures may also be used for comparison of reactivity of the samples. Coal combustion is a heterogeneous reaction and the rate of heat flow is a function of the combustion rate. A lower temperature for the maximum heat flow rate is a consequence of a higher combustion rate. Based on this fact, the temperatures of the maximum heat flow given in Table 2 may be used for a definition of a new reactivity order. A comparison of these temperatures for the raw lignites and its pretreated forms gives different reactivity pattern depending on the types of lignites. For example, raw Kangal lignite seems to be more reactive than its oxidized or demineralized forms. In contrast, raw Go¨lbasi lignite seems to be less reactive than its demineralized form. These results suggest that the effect of the treatments on the rate of heat flows, or on the rates of the carbon–oxygen reactions are not the same for different lignites. Structural changes in solid phase during the treatments, as mentioned earlier, may be an important parameter in this different behavior although several other parameters may also affect the rate of the reactions [2,4,5,23,24]. Ismail and Walker[24] reported that the geometric configuration of the char surface or the nature of C-bonding on the surface is effective in the char–oxygen reactions. Based on the earlier discussion and on the experimental data, it may be concluded that the effects of the treatments on the solid phase structure of the raw lignites are different. Therefore, the reactivity pattern for carbon–oxygen reactions, consequently the heat flow rates, are changed in a different manner for the lignites. Nevertheless, a comparison of the temperatures of maximum heat flow for the raw lignites gives almost the same reactivity pattern with that for the reactivity in the devolatilization. This is an expected result because the devolatilization is the fist step in combustion. 3.4. A kinetic approach The kinetic analysis of nonisothermal pyrolysis, in
K. Ceylan et al. / Fuel 78 (1999) 1109–1116
1113
values [15,16]. Inserting this definition into Eq. (4) gives da=dt A 2E 6 exp 1 2 an q RT The order of reaction n can be determined by taking logarithm of this equation and then plotting ln {(da /dT)/ (1 2 a ) n} versus 1/T. A rectilinear plot may be obtained with a correct value for n. Coats and Redfern [25] have integrated Eq. (6) by expanding it into series with the boundary conditions of a 0 for T T0 and a a for T T. For the case of n ± 1, they obtained the following equation by ignoring the higher order terms of the series, 1 2 1 2 a 12n AR 2RT 2E 12 exp 7 qE E RT 1 2 nT 2 Eq. (7) may be further simplified if it is assumed 2RT p E. Based on these assumptions, the following equation may be used for estimating the kinetic parameters from TGA data: Fig. 4. Determination of the kinetic parameters by using TGA data for the high temperature region. W: raw Tuncbilek lignite, L: raw Beypazari lignite, O: oxidized Go¨lbasi lignite, X: demineralized Kangal lignite, K: raw Go¨lbasi lignite, (the n values for each sample are given in Table 4).
ln
ln
k Aexp
2E RT
T T0 1 qt
1 2 3
where a is the fraction decomposed, f(a ) a function of the degree of the reaction, t the reaction time, k the rate constant, T absolute temperature, E and A are the activation energy and pre-exponential factor, respectively, R is the gas constant, q the heating rate and T0 the starting temperature. By combining Eqs. 1–3, the decomposition rate may be expressed as: da A 2E f a exp 4 dt q RT In the analysis of TGA data, several definitions have been used for the fractional weight loss, a , but in this work, a is defined as in the same way with Elder and Harris [15]:
a
1 2 ft 1 2 f∞
5
where ft and f∞ are the instantaneous and the final values of the weight fractions, respectively. A graph of a versus temperature gives a modified TGA plot which can be used for kinetic analysis. Based on recent calculations, f(a ) is defined as f(a ) (1 2 a ) n (n is overall reaction order) with singular E and A
8
With the same assumptions as given before, the integration of Eq. (6) for the first order reaction (n 1) gives:
general, is based on the following three equations [9]: da kf a dt
1 2 1 2 a 12n AR E 2 ln 2 qE RT 1 2 nT
2ln 1 2 a AR E 2 ln qE RT T2
9
By using the a values, a plot of left side of Eqs. (8) or (9) versus 1/T should give straight lines for a correct value of n. The conventional Arrhenius parameters, A and E, can then be estimated from the intercept and the slope of these lines. However, a straight line with a high correlation coefficient could not obtained in this study, when the data of whole temperature range were processed together, even if the weight changes below 1008C (this amounted to approximately 1%–10% of the total weight change) has been neglected in the evaluation of a values. The shape of TGA traces in Fig. 1 suggest that the weight loss rates and the prevailing reaction mechanism change depending on the temperature regions. Instead of the estimation of the overall kinetic parameters for the whole temperature range, therefore, it seems be more convenient to apply Eqs. (8) or (9) for definite temperature intervals. Accordingly, the effective kinetic parameters, in this work, are estimated for two separate temperature regions. The first is the lower temperature region (T , 2008C) where mainly the moisture water is removed and the second is the higher temperature region (T . 3008C) where generally 80% of the conversion is took place. The two-stage kinetics is observed also by many other workers [8,16,17]. In the evaluation of Eq. (8), the best value for n was searched by a computer program in the range from 0.2 to 2.0 to get a straight line. The n value giving a straight line with a highest correlation coefficient was assumed to be the order of the overall pyrolysis reactions. The values of E and A were then calculated from the slope and the intercept of the line, respectively. A regression
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K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Table 3 The pyrolysis kinetic parameters estimated from TGA data (heating rate 208C min 21) a Low temp. (T , 2008C) region
High temp. (T . 3008C) region
Lignite sample
E
A
n
R
E
A
n
R
Raw Kangal Oxidized Kangal Dem. Kangal
34.5 19.2 19.8
0.37 × 10 4 1.15 × 10 6 1.21 × 10 4
1.4 1.6 1.6
0.995 0.975 0.995
71.5 57.5 38.5
0.56 × 10 4 1.54 × 10 5 1.54 × 10 6
1.6 1.8 1.8
0.993 0.983 0.980
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
16.8 15.6 21.2
0.91 × 10 6 0.77 × 10 6 1.23 × 10 6
1.4 1.6 1.6
0.985 0.980 0.990
47.5 57.5 49.6
2.45 × 10 5 7.70 × 10 5 2.75 × 10 5
1.8 1.8 1.8
0.993 0.983 0.990
Raw Beypazari Oxidized Beypazari Dem. Beypazari
32.9 22.3 15.2
0.87 × 10 4 0.29 × 10 4 5.45 × 10 4
1.2 1.6 1.4
0.990 0.980 0.975
85.3 47.5 47.5
5.67 × 10 4 8.53 × 10 5 1.92 × 10 4
1.8 1.8 1.8
0.989 0.980 0.982
Raw Tuncbilek
39.2
0.28 × 10 4
1.4
0.992
88.9
3.13 × 10 5
1.6
0.990
a
21
21
E: energy of activation (kJ mol ), A: pre-exponential factor for Arrhenius statement (s ), n: overall reaction order, R: correlation coefficient.
analysis with “the least squares method” was employed in the estimation of n, E and A for each temperature region. Some representative plots of Eq. (8) for the high temperature region are presented in Fig. 4 for the selected samples. Similar plots are obtained also for the other samples, for both low and high temperature regions. The n values and resulting global kinetic parameters (i.e. A and E values) calculated from these plots are summarized in Table 3. Based on the results of the analyses with a heating rate of 408C min 21, it was concluded that the heating rates have relatively small effect on the kinetic parameters. The data in Table 3 indicate that the weight losses may generally be represented by Eq. (8) with the n values in the range 1.2–1.8, depending on the type of sample. The data in the literature also indicate that the overall reaction order for the nonisothermal decomposition of coals varies from 1.0 to 2.5. For example, Skala et al. [8] reported that TGA data of an oil shale fit the reaction order of 1.0, but, Elder and Harris [15] reported that TGA data of Kentucky coals fit the reaction order of 1.5–2.5. Similarly, the reported activation energies vary in a wide range depending on the type of sample. It seen from the table that the activation energies for the low temperature region are relatively low. This result suggests that the devolatilization process in this region is essentially diffusion controlled. This is an expected result as the weight loses in this region essentially originates from the removal of moisture water. The energies for the oxidized or demineralized samples are generally lower than for the parent raw lignites. As the treated lignites are low moisture content, this result implies that the energies may depend on the initial moisture content. However, the structural changes in solid phase, as discussed earlier, may also affect the activation energies if the process is diffusion controlled. For the higher temperature region, the activation energies are appreciably higher and this suggests that the devolatilization be, essentially, chemically controlled. The estimated values for E and A change appreciably depending on the types of the
samples. The data given in the literature for the high temperature region indicate that the estimated energy values change in the range of 50–200 kJ mol 21, depending on the type of sample [11,14,15,17]. In this respect, the values reported here are in accord with those given in the literature. For example, Ko¨k and Okandan [26] reported activation energies of 60–80 kJ mol 21 for some Turkish lignites. Haddadin and Tawarah [16], reported activation energies of 16–20 kJ mol 21 for the low temperature range, but 50– 80 kJ mol 21 for the high temperature region for the nonisothermal pyrolysis of Jordan oil shale. Elder and Harris [15] reported higher activation energies for bituminous coals. Similarly, the reported pre-exponential factors also vary in the range of 10 3 to 10 14 s 21. The data in Table 3 also indicate that either oxidized or demineralized lignites exhibit appreciably lower activation energies in comparison to those of parent lignites. This result suggest that both oxidation and demineralization processes increase the reactivity of the lignites for the devolatilization. Kudynska and Buckmaster [22] suggested that some peroxides are formed during oxidation and –O–O– bonds in these structures easily undergo unimolecular homolysis even at low temperatures. Therefore, the primary carbonization of the oxidized samples starts at relatively lower temperatures and requires lower energy of activation. Similarly, the acid treatments during the demineralization process cause the cleavage several types of bonding [27]. These type effects may result in lower activation energies for the devolatilization process. In fact, the energies estimated from the thermogravimetric analyses are essentially the effective energies for the sum of different reactions that occur simultaneously. The real activation energies for several reactions may be much higher. DTA or DSC data also enable to estimate the effective kinetic parameters [7–9,16]. Haddadin and Tawarah [16] used directly the differential form of the nth order rate model (Eq. (6)) to estimate the kinetic parameters for each
K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Fig. 5. Plots of nth order equation for the first endotherm of DSC curve of raw Kangal lignite.
peak region in the DTA or DSC curves. A straight line should be obtained by plotting ln{da /dT/(1 2 a ) n} versus 1/T if the order n is known for the peak regions. The correct n value, in this work, was searched by a computer program as mentioned earlier. A relatively different definition of the conversion fraction, a (as is demonstrated in Fig. 3), has been used in the evaluation of DSC and DTA data. In this definition, the value of a ranges from 0.0 to 1.0 at each peak region. The plots of nth order rate model for the first endotherm in DSC curve of raw Kangal lignite and for the second endotherm in DTA curve of demineralized Kangal lignite are given in Figs. 5 and 6, respectively. Similar plots were obtained from the peaks in the DSC and DTA traces of the all samples. The results indicated that generally firstorder reaction model gives straight lines with high correlation coefficients. The kinetic parameters A and E are then estimated from the intercept and the slope of these lines. The estimated activation energies by this method for the pyrolysis (from DTA data) and for the combustion (from DSC data) are summarized in Table 4. A comparison of the data given in Tables 3 and 4
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indicates that the activation energies estimated from TGA, DSC and DTA for the low temperature region are in a good agreement. These results indicate that the gaseous atmosphere has almost no effects on the evaporation of moisture water in this temperature region. However, for the higher temperature region, the type of gaseous atmosphere is an effective parameter. The DSC derived energy values in Table 4 are for the combustion reactions and vary approximately from 50 to 60 kJ mol 21. The TGA or DTA derived values are for the pyrolysis reactions in inert atmosphere. Although the TGA derived energy values vary from approximately 40 to 85 kJ mol 21, the DTA derived values vary from approximately 170 to 260 kJ mol 21. The results indicate that the activation energies estimated from the different methods (i.e. TGA and DTA) are significantly different and DTA derived energy values 2–3 times higher than those derived from TGA. Similar results are reported also by other researchers [8]. This situation essentially originates from the differences in basic data obtained by these methods. In TGA, the weight losses; but in DTA, the temperature changes are registered versus temperature or time. For a simple physical or chemical process, the data obtained from the different methods can easily be correlated. However, for the case of coal pyrolysis, the correlations between the evolution of volatiles and the temperature changes or the heat absorptions are very complicated. The aforementioned results imply that a substantial portion of the pyrolysis reactions at higher temperatures is accompanied by temperature changes. Some of these reactions are also accompanied by weight loss, but some are not. The reactions that produce volatile products (liquids or gases) seem to have lower activation energies in comparison to those reactions that do not produce volatile products.
4. Conclusions The thermogravimetric data discussed earlier suggest that
Fig. 6. Plots of nth order equation for second endotherm of DTA curve of demineralized Kangal lignite.
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K. Ceylan et al. / Fuel 78 (1999) 1109–1116
Table 4 The first order kinetic parameters estimated from DSC data (in air atmosphere) and DTA data (in nitrogen atmosphere) (heating rate 208C min 21) a DSC data
DTA data
Endothermic peak exothermic peak second endotherm
Exothermic peak
Second endotherm
Lignite sample
E
A
E
A
E
A
Raw Kangal Oxidized Kangal Dem. Kangal
28.6 23.9 23.6
1.00 × 10 3 1.12 × 10 3 1.21 × 10 3
52.6 55.3 55.6
2.83 × 10 3 4.21 × 10 3 2.30 × 10 3
198.6 230.0 225.4
1.38 × 10 12 3.73 × 10 12 2.67 × 10 12
Raw Go¨lbasi Oxidized Go¨lbasi Dem. Go¨lbasi
19.6 21.6 21.8
0.14 × 10 3 0.52 × 10 3 1.04 × 10 3
51.6 60.3 50.2
1.56 × 10 3 8.18 × 10 3 1.26 × 10 3
236.1 226.5 177.3
5.08 × 10 13 12.60 × 10 13 3.97 × 10 10
Raw Beypazari Oxidized Beypazari Dem. Beypazari
28.1 — —
0.61 × 10 3 — —
56.3 — —
5.10 × 10 3 — —
258.6 189.8 256.2
1.14 × 10 14 8.87 × 10 12 1.85 × 10 15
Raw Tuncbilek
29.1
0.85 × 10 4
56.4
5.41 × 10 4
168.4
0.47 × 10 11
a
See Tables 1 and 3 for the nomenclature.
the pyrolysis process of the lignites involve at least two kinetically active steps. The major devolatilization starts after 3008C–3508C and the order of overall reaction changes depending on the type of sample and temperature region. The weight loss process is essentially diffusion controlled at the lower temperature region, but is chemically controlled at the higher temperature region. The effective activation energies estimated by different methods for the lower temperature region are similar but the energies for the higher temperature region are appreciably different. The DTA estimated energies are 2–3 times higher than the energies estimated from TGA. This result implies that the most of the higher temperature reactions that do not result in appreciable weight loss proceed with higher activation energies. The activation energies, or the peak temperatures for devolatilization or for ignition of the lignites suggest that the pretreatments such as oxidation or demineralization generally increase the reactivity for devolatilization or for combustion. This result implies that the pretreatments appreciably affect the solid phase structure.
Acknowledgements It is a pleasure to acknowledge the support of this work ¨ .A.F.96/14) by the Research Foundation of (Project No: I.U Ino¨nu¨ University.
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