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A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars
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Proceedings of the Combustion Institute 33 (2011) 1755–1762
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A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars Kongvui Yip a,b, Esther Ng b,1, Chun-Zhu Li a,b, Jun-Ichiro Hayashi c, Hongwei Wu a,b,⇑ a
Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia b Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia c Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, 816-8580, Japan Available online 24 September 2010
Abstract This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Char oxidation; Mechanism; Kinetics; Selective oxidation; Compensation effect
1. Introduction ⇑ Corresponding
author at: Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia. Fax: +61 8 9266 2681. E-mail address: [email protected] (H. Wu). 1 Present Address: Worley Parsons Services Pty Ltd, 45 St Georges Terrace, Perth WA 6000, Australia.
A mechanistic and kinetic understanding on oxidation of chars is crucial to the utilisation of these carbon materials as an energy source via various technological scenarios [1]. Chars, especially those from low-rank fuels such as lignite and biomass, are known to be of heterogeneous nature [2]. Previous studies on the oxidation of
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.07.073
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brown coal char [3–5] and biomass char [6] showed that the char reactivity changes in a complicated manner throughout the conversion and the char structure greatly affects the reactivity. Recent studies [7,8] also demonstrated the evolution of biomass char carbon structure during steam gasification and the catalytic species affects such an evolution significantly. However, the conventional modelling approach frequently utilises a single set of kinetic parameters for char reactions during conversion [1,9]. This would obviously not be an adequate approach. Char structural ordering during the course of oxidation has been suggested in several previous studies [5,10–13]. Deduction on selective consumption of carbon structure was reached based mainly on inferences from the char reactivity and/or the char optical properties [10,11]. Selective oxidation has been reported in a recent study on brown coal chars [5], with the direct carbon structural evidence from FT-Raman spectroscopy. However, no kinetic analysis was carried out and possible selective oxidation for various other carbon materials has not been investigated. In kinetic studies of char-gas reactions, the kinetic compensation effect has been reported for char with different amounts/types of catalysts [14–16], and various explanations were proposed [14–18]. A recent study [4] has reported the kinetic compensation effect throughout conversion during the low-temperature oxidation of a brown coal char and proposed a plausible explanation on the compensation effect based on carbon structure evolution during conversion. Unfortunately, the study was only on one coal char and there was no direct evidence to prove such evolution of char carbon structure to substantiate their explanation on the compensation effect. To the knowledge of the authors, there has been no systematic study on the evolution of kinetics of char oxidation throughout the oxidation conversion coupled with detailed investigations on the evolution of char carbon structure. It is the objectives of this study to derive the kinetic parameters for char oxidation and at the same time using FT-Raman spectroscopy to provide direct evidence on the evolution of char carbon structure with conversion, eventually leading to elucidation of the low-temperature oxidation mechanisms for various carbon materials and the kinetic compensation effect. 2. Experimental 2.1. Samples The raw samples used include natural graphite, an Australian sub-bituminous coal and an Australian brown coal. The sub-bituminous coal has proximate analysis on dry basis of 56.1% fixed
carbon, 32.0% volatile matter, 11.9% ash and ultimate analysis on dry-ash-free basis of 77.92% C, 4.4% H, 15.38% O, 1.45% N, 0.85% S, while the brown coal has 46.7% fixed carbon, 52.2% volatile matter, 1.1% ash and 70.4% C, 5.4% H, 23.30% O, 0.62% N, 0.28% S. The coal particle size was 90– 106 lm. To study the effect of the coal inherent inorganic species, demineralisation was carried out on the raw coal samples, following the procedures detailed elsewhere [5,19], removing most of the inorganic species from the coals. Hereafter, the demineralised coals are also referred to as ‘demin’ coals in figures and tables. 2.2. Pyrolysis Char samples were prepared from pyrolysis of the coals and graphite using a quartz drop-tube/ fixed-bed reactor housed in a furnace. The detailed configuration of this system can be found elsewhere [19]. Pyrolysis was carried out under a fast-heating condition (103 K s1) at 1000 °C under high purity N2 (purity >99.99%). The coal was fed continuously into the preheated reactor for a feeding period of 15 minutes, and upon completion of feeding the reactor was lifted out of the furnace to cool naturally. As a comparison, some slow-heating pyrolysis experiments were also carried out using the same reactor operated as a fixed-bed reactor. The coal sample pre-loaded in the reactor was heated to 1000 °C at 10 K min1 and held at the temperature for 30 minutes. The char samples were recovered and subject to further characterisations. Hereafter, the sample obtained from pyrolysis of natural graphite is also referred to as ‘graphite’. The results presented in this paper refer to those with the chars from fast-heating experiments, unless otherwise stated. 2.3. Char reactivity measurement and derivation of kinetics parameters Char was subject to isothermal oxidation reactivity measurement, using a thermogravimetric analyser (TGA, model TA SDT-Q600), under conditions in the chemical-reaction-controlled regime. Adoption of the mathematical model developed previously [20] for the char particles shows that the effectiveness factor is virtually 1 and analysis using char samples further ground to smaller particle sizes shows negligible changes in the reactivity profiles; these assure negligible internal diffusion. The detailed reactivity measurement procedure can be found elsewhere [3,19]. In order to minimise the effect of chemisorption of oxygen on the reactivity measurement [21], a gas mixture of 5% O2 in N2 was used. Also this study considers only the reactivity data at char carbon conversion levels between 10 and 80%. The specific reactivity (R, min1) of a char at any instant was calculated from the differential mass loss data
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
(dW/dt) according to R = (1/W) (dW/dt), where W is the mass (dry-ash-free) of the char at any time t (min). Under conditions of constant (e.g. excess) partial pressure of the reactant gas, at a given conversion level, the specific reactivity measured in the TGA can be expressed as: R¼
1 dW ¼ Aapp expðEapp =RT Þ W dt
ð1Þ
where Eapp is the apparent activation energy while the pre-exponential factor Aapp includes a sum of many factors, e.g. char compositions, the number of active sites, which are conversion dependent. In other words, we have Eapp ð2Þ RT where R = Specific reactivity (min1), Aapp = Apparent pre-exponential factor (min1), Eapp = Apparent activation energy (kJ mol1), R = Gas constant (kJ K1mol1), T = Temperature (K). Therefore, to obtain the kinetic parameters at a given conversion level, Arrhenius plots were made by plotting the char reactivity values at the conversion level at different temperatures. The kinetic parameters at various oxidation conversions were obtained from the Arrhenius plots. By taking the values of the slope and intercept, the apparent pre-exponential factor and apparent activation energy can be determined.
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has been deconvoluted into three bands: Gr (1540 cm1), Vl (1465 cm1) and Vr (1380 cm1). These bands represent typical structures in amorphous carbon (especially smaller aromatic ring systems) as well as the semi-circle breathing of aromatic rings. The detailed procedure for the analysis, the FT-Raman peak/band assignment, the data processing and typical examples of curve-deconvolution can be found elsewhere [6,22]. All analyses were repeatable and reproducible, the error bar for each analysis being shown in the FT-Raman results later. 3. Results and discussion
ln R ¼ ln Aapp
3.1.2. Kinetics analysis Figure 2 shows the kinetic parameters for all samples. It can be seen that the kinetic parameters
0.08
Raw brown coal char
-1
FT-Raman Spectroscopy (using a Perkin– Elmer Spectrum GX FT-IR/Raman spectrometer) was employed to probe the carbon structure of char samples collected at various conversions. An InGaAs detector operated at room temperature was used to collect Raman scattering using a back scattering configuration. The excitation Nd:YAG laser wavelength was 1064 nm. The Raman spectra in the range between 800 and 1800 cm1 were curve-fitted using the GRAMS/ 32 AI software (version 6.00) into 10 Gaussian bands. Each band represents a specific type of carbon structure. Therefore, various bands of FTRaman spectroscopy would provide insights into the average energy level of these carbon structures in the bulk char sample analysed. The five main bands found for the samples in the present study are the G, Gr, Vl,, Vr, and D bands. The G band (at the band position 1590 cm1) mainly represents aromatic ring quadrant breathing and the graphite E22g vibration so that the observed G band is mainly due to the aromatic ring systems or the ‘graphitic’ structure. The D (1300 cm1) band represents ‘defect’ structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less than 6 rings. The overlap between the D and G bands
3.1.1. Char reactivity Figure 1 shows the reactivity of chars from various samples at 500 °C (except for graphite where the reactivity was measured at 700 °C). Within the conversion levels studied, the char reactivity generally increases with conversion for all samples. The raw brown coal char and the raw sub-bituminous coal char have significantly higher reactivity than the demineralised coal chars, as expected, mainly due to the catalytic effect of inorganic species in the raw coal chars. Their reactivities increase rapidly with conversion. The reactivity of graphite at 500 °C is much lower than that of all other samples, and in fact its reaction time is so long that its reactivity is not practically measurable at 500 °C. Overall, the reactivity for all samples evolves with the progress of char conversion. To provide further insights into the char reactivity and the reaction mechanism, kinetic analysis was then carried out.
Specific reactivity (min )
2.4. Char carbon structure evolution during oxidation
3.1. Evolution of char carbon structure during oxidation and significance of selective oxidation
0.06 0.04
Demin brown coal char
Raw subbituminous coal char Graphite
0.02 Demin sub-bituminous coal char
0.00 0 10 20 30 40 50 60 70 80 90 100 Char conversion (%)
Fig. 1. Specific oxidation reactivity of various samples at 500 °C (except for graphite where the oxidation temperature was 700 °C).
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-1
Eapp (kJ mol )
Graphite 160
(a)
Demin sub-bituminous char Demin brown char
120
0.65 0.60
0.50
(b)
(b) G band area (fraction of total)
20
ln Aapp
(a)
0.70
0.55
Raw brown char Raw sub-bituminous char
80
15
10
5 0
D band area (fraction of total)
200
20
40
60
80
100
Char conversion (%)
Fig. 2. Kinetic parameters for oxidation of various coal chars and graphite. (a) Eapp; and (b) ln Aapp. Legend: Graphite (s); raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
evolve significantly throughout the conversion, indicating that a single set of kinetic parameters is obviously not adequate to represent the reactivity of all these carbon materials. The change of apparent kinetic parameters with char oxidation conversion is most probably a reflection of a change in the char carbon structure with conversion. Some possible reasons for the change of char structure such as thermal annealing [23] and catalytic char ordering [24] are not expected to be significant under the current low-temperature oxidation condition. Neither are the volatile-char interactions [4], which are also minimal under the current reactivity measurement conditions. Hence, the results in Fig. 2 further suggest that some preferential consumption of the char structure has occurred with progress of conversion during the oxidation. It is clear that the evolution of kinetic parameters above is an indication of possible changes in the char structure. To provide direct evidence on the change of char carbon structure, FT-Raman spectroscopic analysis, which can reveal the carbon structures present in the chars, was therefore carried out. 3.1.3. Evolution of char carbon structure Figure 3 shows the results from FT-Raman spectroscopy for graphite. As expected, D and G have been found to be the major bands for graphite, contributing to a total of >90% of the band area. Generally, D band represents ‘defect’ structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less
0.28 0.26 0.24 0.22 0.20
0
20
40 60 80 Char conversion (%)
100
Fig. 3. Peak areas from FT-Raman spectroscopy as a function of oxidation conversion, for graphite: (a) D; and (b) G.
than six rings while G band represents ‘graphitic’ structure. The results suggest that the graphite sample has a significant proportion of D band structure. Within the D and G bands, the data show that the D band decrease with conversion while G band increases with conversion. Figure 4 shows the results from FT-Raman spectroscopy for various coal chars respectively. D, (Gr+Vl+Vr) and G are the major bands for the various coal chars. (Gr+Vl+Vr) band represents amorphous structure with small aromatic ring systems. It can be seen that, for all cases, the D band increases with conversion, whereas the (Gr+Vl+Vr) band and the (Gr+Vl+Vr)/D ratio decrease with conversion. Therefore, the data in Fig. 3 and Fig. 4 provide the direct evidence that the char carbon structure become increasingly ordered or condensed with conversion. This is in accordance with Fig. 2a which shows a progressive increase in the activation energy with conversion, indicating that the char sample is progressively enriched with carbon structure of higher energy level to be overcome for the oxidation reaction to take place. Therefore, Figs. 2–4, altogether, have confirmed that ‘selective oxidation’ has occurred throughout the conversion for all samples, even for graphite. The results presented so far is from the fastheating pyrolysis experiment where the feeding condition itself can possibly introduce heterogeneity in the char sample, since the continuous feeding of samples led to the production of chars with different holding times in the reactor. Pyrolysis experiments were therefore also carried out under slow-heating conditions with the whole char sample having the same residence time inside
Demin sub-bituminous coal char Demin brown coal char
(a)
0.30 Raw sub-bituminous coal char
0.26
Raw brown coal char
0.22 0.34
Raw brown coal char (b) Demin sub-bituminous coal char
0.32
Raw sub-bituminous coal char
0.04
0.02
Slow-heating pyrolysis
Demin sub-bituminous coal char
1.0 Demin brown coal char
0.8 0.16
(b)
Fast-heating pyrolysis Slow-heating pyrolysis
1.4
(Gr+Vl+Vr)/D
Raw brown coal char (c) Raw sub-bituminous coal char
Fast-heating pyrolysis
0.03
Demin brown coal char
1.2
1.2 1.0
(d) 0.8 40
0.12 0.08
(a)
0.01
0.30 0.28 1.4
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0.05 -1
0.34
Specific reactivity (min )
(Gr+Vl+Vr) band area D band area G band area (fraction of total) (G +V +V )/D (fraction of total) (fraction of total) r r l
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
0
20
40
60
80
100
Char conversion (%)
Fig. 4. Peak areas from FT-Raman spectroscopy as a function of oxidation conversion, for various coal chars: (a) D; (b) Gr+Vl+Vr; (c) Ratio of (Gr+Vl+Vr) to D; and (d) G. Legend: raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
the reactor and hence uniform initial char structure. From Fig. 5a, the reactivity of char from slow-heating pyrolysis is lower than that from fast-heating experiment. This is most probably due to the effect of heating rate [19] and the longer holding time [23] for the slow-heating experiment. Figure 5b shows that ‘selective oxidation’ for the char from slow-heating pyrolysis is still significant throughout conversion, and that the char from the slow-heating pyrolysis generally has a more condensed structure than that from the fast-heating pyrolysis. Interestingly, the behaviour and progress of selective oxidation vary from sample to sample. The discussion is then outlined first for the samples in the absence of catalytic effect on the oxidation process (for graphite and demineralised coal chars) followed by the samples in the presence of catalytic effect (for raw coal chars): (a). In the absence of catalytic effects. Graphite has a much lower reactivity (Fig. 1) and considerably higher activation energy than the demineralised coal chars (Fig. 2a). However, also from Fig. 2a, the extent of evolution of activation energy (indicating the extent of selective oxidation) throughout the conversion appears to be rather similar for the 3 mentioned samples. The evolution of activation energy for the deminera-
50
60 70 80 90 Coal conversion (%)
100
Fig. 5. Comparison between raw sub-bituminous coal chars from fast-heating and slow-heating pyrolysis (a) specific reactivity at 500 °C; and (b) (Gr+Vl+Vr)/D ratio from FT-Raman spectroscopy, as a function of coal conversion.
lised coal chars is rather similar. This is supported by the results from FT- Raman spectroscopy. From Fig. 3 and Fig. 4, for graphite, D and G band areas are higher than those in the demineralised coal chars that also contain significant proportion of carbon structure represented by (Gr+Vl+Vr). This means that graphite has more inert or highly condensed carbon structure than the demineralised sub-bituminous and brown coal chars. The demineralised coal chars generally has rather similar carbon structures throughout the oxidation conversion. Therefore, in the absence of catalytic effect, the extent of carbon graphitisation or degree of coalification process determines the initial char structure following pyrolysis, and subsequently affects the oxidation reactivity considerably but not the extent of selective oxidation appreciably. Note that a progressively condensed structure does not necessarily mean a decrease in overall reactivity (Fig. 1). As the carbon structure becomes more condensed, more active sites can possibly result and hence the active surface area can increase [5,25], in accordance with the observed increase in Aapp with conversion (Fig. 2b), resulting in an increase in the overall reactivity. This will be discussed further later pertaining to the kinetic compensation effect. (b). In the presence of catalytic effects. From Fig. 2 again, the raw sub-bituminous coal char and the raw brown coal char have considerably lower activation energy and a higher extent of selective oxidation than their corresponding demineralised char. From FT-Raman spectroscopy
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
(Fig. 4), generally, each demineralised coal char has more condensed carbon structure initially and also throughout the oxidation conversion, compared to the corresponding raw coal char, as evidenced from D band, (Gr+Vl+Vr) band and (Gr+Vl+Vr)/D ratio. This suggests that the demineralisation process, in addition to removal of inorganic species, have resulted in a more condensed initial char structure prior to oxidation, possibly occurring through the changes to the coal structure following demineralisation [18,26,27] and/or a change in the resultant char structure during pyrolysis [22,27]. It is also possible that, subsequently, for the raw coal chars, the oxidation is more focussed or localised on catalytic sites relative to the reaction on carbon active sites (while for the demineralised coal chars the reaction occurs slowly throughout the char matrix on carbon active sites solely) contributing to a constantly less condensed carbon structure throughout the conversion compared to the demineralised coal chars. This has been shown in recent studies on the effect of catalysts on the oxidation pathway of a brown coal char [5] and the effect of catalysts on the steam gasification of various biomass chars [8]. Therefore, in the presence of catalytic effect (for the raw coal chars), the mechanistic explanation for the lowered apparent activation energy and the higher extent of selective oxidation for char oxidation is most probably: (a) the effect of the catalytic species on the initial char carbon structure following pyrolysis; and (b) the action of the catalytic species during oxidation and its effect on the char carbon structure. 3.2. Kinetic compensation effect Figure 2 also indicates that while Eapp increases with conversion (Fig. 2a), ln Aapp also seems to increase with conversion (Fig. 2b). The data of ln Aapp are then plotted against those of Eapp to provide further insights into the mechanism of oxidation of the various samples. Figure 6 shows the relation between ln Aapp and Eapp for the oxidation of various samples. It can be seen that ln Aapp indeed varies in linear proportion with Eapp, exhibiting the so-called “kinetic compensation effect”, yielding the correlation ‘ln Aapp = mEapp + c’, where m is the proportionality constant and c is the constant at intercept Eapp = 0. Kinetic compensation effect of coal char reactions were reported previously [14–16], concerning the catalytic char gasification reactions involving different amounts/kinds of catalysts. Previous studies [14–16] also typically derived the kinetic data using reactivity data at one particular char conversion level and the char structure was assumed to be similar throughout gasification. These are different to the present study, where the kinetic compensation effect shown in Fig. 6 is evidenced for both non-catalytic and catalytic
2
Graphite (lnAapp=0.2Eapp-18.2, R =0.99 )
25
2
20
Raw brown char (lnAapp=0.2Eapp-5.0, R =0.99)
15
ln Aapp
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Demin sub-bituminous char 2 (lnAapp=0.2Eapp-10.7, R =0.99)
10 5
2
Demin brown char (lnAapp=0.2Eapp-9.7, R =0.99) 2
0 60
Raw sub-bituminous char (lnAapp=0.2Eapp-7.3, R =0.98)
80
100 120 140 160 180 200 -1
Eapp (kJ mol )
Fig. 6. ln Aapp versus Eapp for the oxidation of various coal chars and graphite, R2 being the correlation coefficient. Legend: Graphite (s); raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
reactions throughout the conversion during lowtemperature oxidation. In other words, there is a continuous evolution of kinetic parameters for the reacting char as conversion increases and such evolution follows the kinetic compensation effect, observed for all samples. With the direct information on carbon structure obtained via FT-Raman spectroscopy, the results clearly demonstrate that the compensation effect reported in Fig. 6, i.e. the increase in both Eapp (indicating the average energy level of the carbon structure) and Aapp (largely indicating the number of active sites), is related to the heterogeneous char carbon structure. The data from FT-Raman spectroscopy clearly show that the carbon structure becomes progressively condensed (the ring size progressively increases) with conversion. At low conversions, although the activation energy needed to be overcome for a carbon on a smaller ring system in the char is relatively small, the activation would have a more localised effect over a small number of carbon atoms. This leads to relatively a small increase on the total number of active sites. On the contrary, towards high conversions, a highly condensed aromatic-ring cluster would exhibit high activation energy but the activation of one carbon site on one of such aromatic-ring clusters would possibly lead the activation of many carbon atoms on the condensed aromatic ring clusters. Consequently, an apparent increase in the total number of active sites is observed. Such an explanation was suggested and proposed in a recent study [4]; the FT-Raman spectroscopic analysis in a previous study [5] and the present study has provided further evidence on this explanation. In the compensation effect correlation, the gradient m is the ratio of ln Aapp and Eapp therefore represents the relationship between the increase in the active sites population and the increase in
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
activation energy. In other words, the gradient m can be used as an indication of the nature of the char carbon structural condensation evolution. On the other hand, the initial char carbon structure determines the intercept c. In the absence of catalytic effect (for the graphite sample and the demineralised coal chars), it is observed that the gradient m is similar, whereas the graphite sample has a different intercept c value to the demineralised coal chars. This suggests that although the initial char structures produced from different samples are different as already discussed earlier, the oxidation process appears to follow similar nature for structural condensation till near completion of the reaction. It can also be noted that the extent of selective oxidation (i.e. extent/degree of condensation of carbon structure) is similar, as implied by the two parallel dashed lines in Fig. 6. In other words, both the ‘nature’ and ‘degree/ extent’ of condensation of the carbon structure are similar for various carbon materials in the absence of catalysts. Hence, the initial char carbon structure is the key factor determining the compensation effect here. In the presence of catalytic effect (for the raw coal chars), different intercept c values are obtained in comparison to the demineralised coal chars and graphite, indicating a different char initial structure is obtained. Interestingly, the gradient m value for raw coal chars is similar to other samples (this gradient has been found to be approximately 0.2 here, in close approximation to 0.198 as reported previously [4] for low-temperature oxidation of coal chars prepared from the same brown coal but under different conditions). However, the extent of evolution of activation energy (i.e. the extent of selective oxidation) is rather different between the raw sub-bituminous coal char and the raw brown coal char, and is different to the demineralised coal chars and graphite. This suggests that even though the presence of catalytic species may have altered the reaction pathway during oxidation due to the actions of catalysts affecting the degree/extent of condensation of the carbon structure, yet overall it seems that the apparent oxidation process still follows the intrinsic nature of carbon as in the absence of catalytic species. In other words, the catalytic effects during oxidation seems to be mainly on altering the pathway of evolution/condensation of the carbon structure, yet a certain evolution/ condensation degree of carbon structure will always lead to a emergence of a relatively constant active sites population. In a nutshell, during oxidation, the catalysts result in a similar ‘nature’ but possibly a different ‘degree/extent’ of carbon structural condensation. Therefore, in the presence of catalysts, the effect of the catalysts on the initial char carbon structure, as well as the actions of the catalyst during oxidation, is an important determining factor for the oxidation behaviour.
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The above findings have important implications in kinetics modelling of the coal char oxidation reaction. The existence of the compensation effect and its linear correlation will aid in modelling the char oxidation at various conversion levels. In the absence catalytic effect, from the present study, it appears that the correlation ‘ln Aapp = 0.2 Eapp + c’, with a Eapp change of 23 kJ mol1 from 10 to 80% char conversion, can be adopted for various samples (within the region encompassed by the 2 parallel dashed lines in Fig. 6), provided that the kinetic parameters for the initial char are known. In the presence of catalysts, this correlation is still valid since the overall reaction follows the intrinsic nature of carbon as aforementioned, but extra information on the extent/degree of evolution of activation energy (as results of the oxidation reaction pathway taken) would be needed, apart from the kinetic parameters for the initial char (as results of the initial char carbon structure). Therefore, exploitation of the compensation effect correlation will require more information on the catalysis during the course of oxidation, such as the amount, kind and/or form of catalysts as well as the interactions between the char carbon structure and the catalysts. 4. Conclusions In this paper, a mechanistic and kinetic study has been carried out on low-temperature oxidation of a range of carbon materials. The kinetic parameters evolve continuously throughout the oxidation conversion. Together with the direct evidence from FT-Raman spectroscopy, the results have shown the occurrence of ‘selective oxidation’ for the samples studied including graphite. Parallel to selective oxidation, the present study has also revealed the occurrence of kinetic compensation effect at various char conversion levels, for all samples, due to the heterogeneous nature and continuous evolution of char carbon structure during conversion. Under the current experimental conditions, oxidation of all samples, with or without catalysts, appears to follow the similar nature of structural condensation evolution. In the absence of catalysts, both the ‘nature’ and ‘degree/extent’ of condensation evolution of the carbon structure are similar for various carbon materials, largely dependent on the initial char carbon structure. However in the presence of catalysts, although the ‘nature’ of carbon structural condensation evolution remains similar, the ‘degree/extent’ of such evolution is also affected by the catalysis during the course of oxidation. Acknowledgements The authors gratefully acknowledge the partial support from the Cooperative Research Centre
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for Coal in Sustainable Development (CCSD) during the initial stage of the research work. Some discussion with Dong-ke Zhang is also acknowledged.
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[13] C. Sheng, Fuel 86 (2007) 2316–2324. [14] A.P. Dhupe, A.N. Gokarn, L.K. Doraiswamy, Fuel 70 (1991) 839–844. [15] R.H. Essenhigh, M.K. Misraf, Energy Fuels 4 (1990) 171–177. [16] T.W. Kwon, J.R. Kim, S.D. Kim, W.H. Park, Fuel 68 (1989) 416–421. [17] G.C. Bond, M.A. Keane, H. Kral, J.A. Lercher, Catalysis Review – Science and Engineering 42 (2000) 323–383. [18] L. Liu, Q. Guo, Chem. Rev. 101 (2001) 673–695. [19] K. Yip, H. Wu, D.-k. Zhang, Energy Fuels 21 (2007) 2883–2891. [20] K. Yip, H. Wu, D.-k. Zhang, Proc. Combust. Inst. 32 (2009) 2675–2683. [21] B. Feng, S.K. Bhatia, Chem. Eng. Sci. 57 (2002) 2907–2920. [22] X. Li, J.-i. Hayashi, C.-Z. Li, Fuel 85 (2006) 1700– 1707. [23] N.V. Russel, Thermal Deactivation and Reactivity of High Temperature Coal Chars (PhD Thesis), University of London, London, 1998. [24] B. Feng, S.K. Bhatia, J.C. Barry, Carbon 40 (2002) 481–496. [25] E. Bar-Ziv, I.I. Kantorovich, Prog. Energy Combust. Sci. 27 (2001) 667–697. [26] C. Sathe, Y. Pang, C.-Z. Li, Energy Fuels 13 (1999) 748–755. [27] F. Rubiera, A. Arenillas, C. Pevida, et al., Fuel Processing Technology 79 (2002) 273–279.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 33 (2011) 1755–1762
www.elsevier.com/locate/proci
A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars Kongvui Yip a,b, Esther Ng b,1, Chun-Zhu Li a,b, Jun-Ichiro Hayashi c, Hongwei Wu a,b,⇑ a
Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia b Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia c Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, 816-8580, Japan Available online 24 September 2010
Abstract This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Char oxidation; Mechanism; Kinetics; Selective oxidation; Compensation effect
1. Introduction ⇑ Corresponding
author at: Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia. Fax: +61 8 9266 2681. E-mail address: [email protected] (H. Wu). 1 Present Address: Worley Parsons Services Pty Ltd, 45 St Georges Terrace, Perth WA 6000, Australia.
A mechanistic and kinetic understanding on oxidation of chars is crucial to the utilisation of these carbon materials as an energy source via various technological scenarios [1]. Chars, especially those from low-rank fuels such as lignite and biomass, are known to be of heterogeneous nature [2]. Previous studies on the oxidation of
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.07.073
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brown coal char [3–5] and biomass char [6] showed that the char reactivity changes in a complicated manner throughout the conversion and the char structure greatly affects the reactivity. Recent studies [7,8] also demonstrated the evolution of biomass char carbon structure during steam gasification and the catalytic species affects such an evolution significantly. However, the conventional modelling approach frequently utilises a single set of kinetic parameters for char reactions during conversion [1,9]. This would obviously not be an adequate approach. Char structural ordering during the course of oxidation has been suggested in several previous studies [5,10–13]. Deduction on selective consumption of carbon structure was reached based mainly on inferences from the char reactivity and/or the char optical properties [10,11]. Selective oxidation has been reported in a recent study on brown coal chars [5], with the direct carbon structural evidence from FT-Raman spectroscopy. However, no kinetic analysis was carried out and possible selective oxidation for various other carbon materials has not been investigated. In kinetic studies of char-gas reactions, the kinetic compensation effect has been reported for char with different amounts/types of catalysts [14–16], and various explanations were proposed [14–18]. A recent study [4] has reported the kinetic compensation effect throughout conversion during the low-temperature oxidation of a brown coal char and proposed a plausible explanation on the compensation effect based on carbon structure evolution during conversion. Unfortunately, the study was only on one coal char and there was no direct evidence to prove such evolution of char carbon structure to substantiate their explanation on the compensation effect. To the knowledge of the authors, there has been no systematic study on the evolution of kinetics of char oxidation throughout the oxidation conversion coupled with detailed investigations on the evolution of char carbon structure. It is the objectives of this study to derive the kinetic parameters for char oxidation and at the same time using FT-Raman spectroscopy to provide direct evidence on the evolution of char carbon structure with conversion, eventually leading to elucidation of the low-temperature oxidation mechanisms for various carbon materials and the kinetic compensation effect. 2. Experimental 2.1. Samples The raw samples used include natural graphite, an Australian sub-bituminous coal and an Australian brown coal. The sub-bituminous coal has proximate analysis on dry basis of 56.1% fixed
carbon, 32.0% volatile matter, 11.9% ash and ultimate analysis on dry-ash-free basis of 77.92% C, 4.4% H, 15.38% O, 1.45% N, 0.85% S, while the brown coal has 46.7% fixed carbon, 52.2% volatile matter, 1.1% ash and 70.4% C, 5.4% H, 23.30% O, 0.62% N, 0.28% S. The coal particle size was 90– 106 lm. To study the effect of the coal inherent inorganic species, demineralisation was carried out on the raw coal samples, following the procedures detailed elsewhere [5,19], removing most of the inorganic species from the coals. Hereafter, the demineralised coals are also referred to as ‘demin’ coals in figures and tables. 2.2. Pyrolysis Char samples were prepared from pyrolysis of the coals and graphite using a quartz drop-tube/ fixed-bed reactor housed in a furnace. The detailed configuration of this system can be found elsewhere [19]. Pyrolysis was carried out under a fast-heating condition (103 K s1) at 1000 °C under high purity N2 (purity >99.99%). The coal was fed continuously into the preheated reactor for a feeding period of 15 minutes, and upon completion of feeding the reactor was lifted out of the furnace to cool naturally. As a comparison, some slow-heating pyrolysis experiments were also carried out using the same reactor operated as a fixed-bed reactor. The coal sample pre-loaded in the reactor was heated to 1000 °C at 10 K min1 and held at the temperature for 30 minutes. The char samples were recovered and subject to further characterisations. Hereafter, the sample obtained from pyrolysis of natural graphite is also referred to as ‘graphite’. The results presented in this paper refer to those with the chars from fast-heating experiments, unless otherwise stated. 2.3. Char reactivity measurement and derivation of kinetics parameters Char was subject to isothermal oxidation reactivity measurement, using a thermogravimetric analyser (TGA, model TA SDT-Q600), under conditions in the chemical-reaction-controlled regime. Adoption of the mathematical model developed previously [20] for the char particles shows that the effectiveness factor is virtually 1 and analysis using char samples further ground to smaller particle sizes shows negligible changes in the reactivity profiles; these assure negligible internal diffusion. The detailed reactivity measurement procedure can be found elsewhere [3,19]. In order to minimise the effect of chemisorption of oxygen on the reactivity measurement [21], a gas mixture of 5% O2 in N2 was used. Also this study considers only the reactivity data at char carbon conversion levels between 10 and 80%. The specific reactivity (R, min1) of a char at any instant was calculated from the differential mass loss data
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
(dW/dt) according to R = (1/W) (dW/dt), where W is the mass (dry-ash-free) of the char at any time t (min). Under conditions of constant (e.g. excess) partial pressure of the reactant gas, at a given conversion level, the specific reactivity measured in the TGA can be expressed as: R¼
1 dW ¼ Aapp expðEapp =RT Þ W dt
ð1Þ
where Eapp is the apparent activation energy while the pre-exponential factor Aapp includes a sum of many factors, e.g. char compositions, the number of active sites, which are conversion dependent. In other words, we have Eapp ð2Þ RT where R = Specific reactivity (min1), Aapp = Apparent pre-exponential factor (min1), Eapp = Apparent activation energy (kJ mol1), R = Gas constant (kJ K1mol1), T = Temperature (K). Therefore, to obtain the kinetic parameters at a given conversion level, Arrhenius plots were made by plotting the char reactivity values at the conversion level at different temperatures. The kinetic parameters at various oxidation conversions were obtained from the Arrhenius plots. By taking the values of the slope and intercept, the apparent pre-exponential factor and apparent activation energy can be determined.
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has been deconvoluted into three bands: Gr (1540 cm1), Vl (1465 cm1) and Vr (1380 cm1). These bands represent typical structures in amorphous carbon (especially smaller aromatic ring systems) as well as the semi-circle breathing of aromatic rings. The detailed procedure for the analysis, the FT-Raman peak/band assignment, the data processing and typical examples of curve-deconvolution can be found elsewhere [6,22]. All analyses were repeatable and reproducible, the error bar for each analysis being shown in the FT-Raman results later. 3. Results and discussion
ln R ¼ ln Aapp
3.1.2. Kinetics analysis Figure 2 shows the kinetic parameters for all samples. It can be seen that the kinetic parameters
0.08
Raw brown coal char
-1
FT-Raman Spectroscopy (using a Perkin– Elmer Spectrum GX FT-IR/Raman spectrometer) was employed to probe the carbon structure of char samples collected at various conversions. An InGaAs detector operated at room temperature was used to collect Raman scattering using a back scattering configuration. The excitation Nd:YAG laser wavelength was 1064 nm. The Raman spectra in the range between 800 and 1800 cm1 were curve-fitted using the GRAMS/ 32 AI software (version 6.00) into 10 Gaussian bands. Each band represents a specific type of carbon structure. Therefore, various bands of FTRaman spectroscopy would provide insights into the average energy level of these carbon structures in the bulk char sample analysed. The five main bands found for the samples in the present study are the G, Gr, Vl,, Vr, and D bands. The G band (at the band position 1590 cm1) mainly represents aromatic ring quadrant breathing and the graphite E22g vibration so that the observed G band is mainly due to the aromatic ring systems or the ‘graphitic’ structure. The D (1300 cm1) band represents ‘defect’ structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less than 6 rings. The overlap between the D and G bands
3.1.1. Char reactivity Figure 1 shows the reactivity of chars from various samples at 500 °C (except for graphite where the reactivity was measured at 700 °C). Within the conversion levels studied, the char reactivity generally increases with conversion for all samples. The raw brown coal char and the raw sub-bituminous coal char have significantly higher reactivity than the demineralised coal chars, as expected, mainly due to the catalytic effect of inorganic species in the raw coal chars. Their reactivities increase rapidly with conversion. The reactivity of graphite at 500 °C is much lower than that of all other samples, and in fact its reaction time is so long that its reactivity is not practically measurable at 500 °C. Overall, the reactivity for all samples evolves with the progress of char conversion. To provide further insights into the char reactivity and the reaction mechanism, kinetic analysis was then carried out.
Specific reactivity (min )
2.4. Char carbon structure evolution during oxidation
3.1. Evolution of char carbon structure during oxidation and significance of selective oxidation
0.06 0.04
Demin brown coal char
Raw subbituminous coal char Graphite
0.02 Demin sub-bituminous coal char
0.00 0 10 20 30 40 50 60 70 80 90 100 Char conversion (%)
Fig. 1. Specific oxidation reactivity of various samples at 500 °C (except for graphite where the oxidation temperature was 700 °C).
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K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762 0.75
-1
Eapp (kJ mol )
Graphite 160
(a)
Demin sub-bituminous char Demin brown char
120
0.65 0.60
0.50
(b)
(b) G band area (fraction of total)
20
ln Aapp
(a)
0.70
0.55
Raw brown char Raw sub-bituminous char
80
15
10
5 0
D band area (fraction of total)
200
20
40
60
80
100
Char conversion (%)
Fig. 2. Kinetic parameters for oxidation of various coal chars and graphite. (a) Eapp; and (b) ln Aapp. Legend: Graphite (s); raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
evolve significantly throughout the conversion, indicating that a single set of kinetic parameters is obviously not adequate to represent the reactivity of all these carbon materials. The change of apparent kinetic parameters with char oxidation conversion is most probably a reflection of a change in the char carbon structure with conversion. Some possible reasons for the change of char structure such as thermal annealing [23] and catalytic char ordering [24] are not expected to be significant under the current low-temperature oxidation condition. Neither are the volatile-char interactions [4], which are also minimal under the current reactivity measurement conditions. Hence, the results in Fig. 2 further suggest that some preferential consumption of the char structure has occurred with progress of conversion during the oxidation. It is clear that the evolution of kinetic parameters above is an indication of possible changes in the char structure. To provide direct evidence on the change of char carbon structure, FT-Raman spectroscopic analysis, which can reveal the carbon structures present in the chars, was therefore carried out. 3.1.3. Evolution of char carbon structure Figure 3 shows the results from FT-Raman spectroscopy for graphite. As expected, D and G have been found to be the major bands for graphite, contributing to a total of >90% of the band area. Generally, D band represents ‘defect’ structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less
0.28 0.26 0.24 0.22 0.20
0
20
40 60 80 Char conversion (%)
100
Fig. 3. Peak areas from FT-Raman spectroscopy as a function of oxidation conversion, for graphite: (a) D; and (b) G.
than six rings while G band represents ‘graphitic’ structure. The results suggest that the graphite sample has a significant proportion of D band structure. Within the D and G bands, the data show that the D band decrease with conversion while G band increases with conversion. Figure 4 shows the results from FT-Raman spectroscopy for various coal chars respectively. D, (Gr+Vl+Vr) and G are the major bands for the various coal chars. (Gr+Vl+Vr) band represents amorphous structure with small aromatic ring systems. It can be seen that, for all cases, the D band increases with conversion, whereas the (Gr+Vl+Vr) band and the (Gr+Vl+Vr)/D ratio decrease with conversion. Therefore, the data in Fig. 3 and Fig. 4 provide the direct evidence that the char carbon structure become increasingly ordered or condensed with conversion. This is in accordance with Fig. 2a which shows a progressive increase in the activation energy with conversion, indicating that the char sample is progressively enriched with carbon structure of higher energy level to be overcome for the oxidation reaction to take place. Therefore, Figs. 2–4, altogether, have confirmed that ‘selective oxidation’ has occurred throughout the conversion for all samples, even for graphite. The results presented so far is from the fastheating pyrolysis experiment where the feeding condition itself can possibly introduce heterogeneity in the char sample, since the continuous feeding of samples led to the production of chars with different holding times in the reactor. Pyrolysis experiments were therefore also carried out under slow-heating conditions with the whole char sample having the same residence time inside
Demin sub-bituminous coal char Demin brown coal char
(a)
0.30 Raw sub-bituminous coal char
0.26
Raw brown coal char
0.22 0.34
Raw brown coal char (b) Demin sub-bituminous coal char
0.32
Raw sub-bituminous coal char
0.04
0.02
Slow-heating pyrolysis
Demin sub-bituminous coal char
1.0 Demin brown coal char
0.8 0.16
(b)
Fast-heating pyrolysis Slow-heating pyrolysis
1.4
(Gr+Vl+Vr)/D
Raw brown coal char (c) Raw sub-bituminous coal char
Fast-heating pyrolysis
0.03
Demin brown coal char
1.2
1.2 1.0
(d) 0.8 40
0.12 0.08
(a)
0.01
0.30 0.28 1.4
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0.05 -1
0.34
Specific reactivity (min )
(Gr+Vl+Vr) band area D band area G band area (fraction of total) (G +V +V )/D (fraction of total) (fraction of total) r r l
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
0
20
40
60
80
100
Char conversion (%)
Fig. 4. Peak areas from FT-Raman spectroscopy as a function of oxidation conversion, for various coal chars: (a) D; (b) Gr+Vl+Vr; (c) Ratio of (Gr+Vl+Vr) to D; and (d) G. Legend: raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
the reactor and hence uniform initial char structure. From Fig. 5a, the reactivity of char from slow-heating pyrolysis is lower than that from fast-heating experiment. This is most probably due to the effect of heating rate [19] and the longer holding time [23] for the slow-heating experiment. Figure 5b shows that ‘selective oxidation’ for the char from slow-heating pyrolysis is still significant throughout conversion, and that the char from the slow-heating pyrolysis generally has a more condensed structure than that from the fast-heating pyrolysis. Interestingly, the behaviour and progress of selective oxidation vary from sample to sample. The discussion is then outlined first for the samples in the absence of catalytic effect on the oxidation process (for graphite and demineralised coal chars) followed by the samples in the presence of catalytic effect (for raw coal chars): (a). In the absence of catalytic effects. Graphite has a much lower reactivity (Fig. 1) and considerably higher activation energy than the demineralised coal chars (Fig. 2a). However, also from Fig. 2a, the extent of evolution of activation energy (indicating the extent of selective oxidation) throughout the conversion appears to be rather similar for the 3 mentioned samples. The evolution of activation energy for the deminera-
50
60 70 80 90 Coal conversion (%)
100
Fig. 5. Comparison between raw sub-bituminous coal chars from fast-heating and slow-heating pyrolysis (a) specific reactivity at 500 °C; and (b) (Gr+Vl+Vr)/D ratio from FT-Raman spectroscopy, as a function of coal conversion.
lised coal chars is rather similar. This is supported by the results from FT- Raman spectroscopy. From Fig. 3 and Fig. 4, for graphite, D and G band areas are higher than those in the demineralised coal chars that also contain significant proportion of carbon structure represented by (Gr+Vl+Vr). This means that graphite has more inert or highly condensed carbon structure than the demineralised sub-bituminous and brown coal chars. The demineralised coal chars generally has rather similar carbon structures throughout the oxidation conversion. Therefore, in the absence of catalytic effect, the extent of carbon graphitisation or degree of coalification process determines the initial char structure following pyrolysis, and subsequently affects the oxidation reactivity considerably but not the extent of selective oxidation appreciably. Note that a progressively condensed structure does not necessarily mean a decrease in overall reactivity (Fig. 1). As the carbon structure becomes more condensed, more active sites can possibly result and hence the active surface area can increase [5,25], in accordance with the observed increase in Aapp with conversion (Fig. 2b), resulting in an increase in the overall reactivity. This will be discussed further later pertaining to the kinetic compensation effect. (b). In the presence of catalytic effects. From Fig. 2 again, the raw sub-bituminous coal char and the raw brown coal char have considerably lower activation energy and a higher extent of selective oxidation than their corresponding demineralised char. From FT-Raman spectroscopy
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
(Fig. 4), generally, each demineralised coal char has more condensed carbon structure initially and also throughout the oxidation conversion, compared to the corresponding raw coal char, as evidenced from D band, (Gr+Vl+Vr) band and (Gr+Vl+Vr)/D ratio. This suggests that the demineralisation process, in addition to removal of inorganic species, have resulted in a more condensed initial char structure prior to oxidation, possibly occurring through the changes to the coal structure following demineralisation [18,26,27] and/or a change in the resultant char structure during pyrolysis [22,27]. It is also possible that, subsequently, for the raw coal chars, the oxidation is more focussed or localised on catalytic sites relative to the reaction on carbon active sites (while for the demineralised coal chars the reaction occurs slowly throughout the char matrix on carbon active sites solely) contributing to a constantly less condensed carbon structure throughout the conversion compared to the demineralised coal chars. This has been shown in recent studies on the effect of catalysts on the oxidation pathway of a brown coal char [5] and the effect of catalysts on the steam gasification of various biomass chars [8]. Therefore, in the presence of catalytic effect (for the raw coal chars), the mechanistic explanation for the lowered apparent activation energy and the higher extent of selective oxidation for char oxidation is most probably: (a) the effect of the catalytic species on the initial char carbon structure following pyrolysis; and (b) the action of the catalytic species during oxidation and its effect on the char carbon structure. 3.2. Kinetic compensation effect Figure 2 also indicates that while Eapp increases with conversion (Fig. 2a), ln Aapp also seems to increase with conversion (Fig. 2b). The data of ln Aapp are then plotted against those of Eapp to provide further insights into the mechanism of oxidation of the various samples. Figure 6 shows the relation between ln Aapp and Eapp for the oxidation of various samples. It can be seen that ln Aapp indeed varies in linear proportion with Eapp, exhibiting the so-called “kinetic compensation effect”, yielding the correlation ‘ln Aapp = mEapp + c’, where m is the proportionality constant and c is the constant at intercept Eapp = 0. Kinetic compensation effect of coal char reactions were reported previously [14–16], concerning the catalytic char gasification reactions involving different amounts/kinds of catalysts. Previous studies [14–16] also typically derived the kinetic data using reactivity data at one particular char conversion level and the char structure was assumed to be similar throughout gasification. These are different to the present study, where the kinetic compensation effect shown in Fig. 6 is evidenced for both non-catalytic and catalytic
2
Graphite (lnAapp=0.2Eapp-18.2, R =0.99 )
25
2
20
Raw brown char (lnAapp=0.2Eapp-5.0, R =0.99)
15
ln Aapp
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Demin sub-bituminous char 2 (lnAapp=0.2Eapp-10.7, R =0.99)
10 5
2
Demin brown char (lnAapp=0.2Eapp-9.7, R =0.99) 2
0 60
Raw sub-bituminous char (lnAapp=0.2Eapp-7.3, R =0.98)
80
100 120 140 160 180 200 -1
Eapp (kJ mol )
Fig. 6. ln Aapp versus Eapp for the oxidation of various coal chars and graphite, R2 being the correlation coefficient. Legend: Graphite (s); raw sub-bituminous coal char (j); demineralised sub-bituminous coal char (h); raw brown coal char (N); and demineralised brown coal char (4).
reactions throughout the conversion during lowtemperature oxidation. In other words, there is a continuous evolution of kinetic parameters for the reacting char as conversion increases and such evolution follows the kinetic compensation effect, observed for all samples. With the direct information on carbon structure obtained via FT-Raman spectroscopy, the results clearly demonstrate that the compensation effect reported in Fig. 6, i.e. the increase in both Eapp (indicating the average energy level of the carbon structure) and Aapp (largely indicating the number of active sites), is related to the heterogeneous char carbon structure. The data from FT-Raman spectroscopy clearly show that the carbon structure becomes progressively condensed (the ring size progressively increases) with conversion. At low conversions, although the activation energy needed to be overcome for a carbon on a smaller ring system in the char is relatively small, the activation would have a more localised effect over a small number of carbon atoms. This leads to relatively a small increase on the total number of active sites. On the contrary, towards high conversions, a highly condensed aromatic-ring cluster would exhibit high activation energy but the activation of one carbon site on one of such aromatic-ring clusters would possibly lead the activation of many carbon atoms on the condensed aromatic ring clusters. Consequently, an apparent increase in the total number of active sites is observed. Such an explanation was suggested and proposed in a recent study [4]; the FT-Raman spectroscopic analysis in a previous study [5] and the present study has provided further evidence on this explanation. In the compensation effect correlation, the gradient m is the ratio of ln Aapp and Eapp therefore represents the relationship between the increase in the active sites population and the increase in
K. Yip et al. / Proceedings of the Combustion Institute 33 (2011) 1755–1762
activation energy. In other words, the gradient m can be used as an indication of the nature of the char carbon structural condensation evolution. On the other hand, the initial char carbon structure determines the intercept c. In the absence of catalytic effect (for the graphite sample and the demineralised coal chars), it is observed that the gradient m is similar, whereas the graphite sample has a different intercept c value to the demineralised coal chars. This suggests that although the initial char structures produced from different samples are different as already discussed earlier, the oxidation process appears to follow similar nature for structural condensation till near completion of the reaction. It can also be noted that the extent of selective oxidation (i.e. extent/degree of condensation of carbon structure) is similar, as implied by the two parallel dashed lines in Fig. 6. In other words, both the ‘nature’ and ‘degree/ extent’ of condensation of the carbon structure are similar for various carbon materials in the absence of catalysts. Hence, the initial char carbon structure is the key factor determining the compensation effect here. In the presence of catalytic effect (for the raw coal chars), different intercept c values are obtained in comparison to the demineralised coal chars and graphite, indicating a different char initial structure is obtained. Interestingly, the gradient m value for raw coal chars is similar to other samples (this gradient has been found to be approximately 0.2 here, in close approximation to 0.198 as reported previously [4] for low-temperature oxidation of coal chars prepared from the same brown coal but under different conditions). However, the extent of evolution of activation energy (i.e. the extent of selective oxidation) is rather different between the raw sub-bituminous coal char and the raw brown coal char, and is different to the demineralised coal chars and graphite. This suggests that even though the presence of catalytic species may have altered the reaction pathway during oxidation due to the actions of catalysts affecting the degree/extent of condensation of the carbon structure, yet overall it seems that the apparent oxidation process still follows the intrinsic nature of carbon as in the absence of catalytic species. In other words, the catalytic effects during oxidation seems to be mainly on altering the pathway of evolution/condensation of the carbon structure, yet a certain evolution/ condensation degree of carbon structure will always lead to a emergence of a relatively constant active sites population. In a nutshell, during oxidation, the catalysts result in a similar ‘nature’ but possibly a different ‘degree/extent’ of carbon structural condensation. Therefore, in the presence of catalysts, the effect of the catalysts on the initial char carbon structure, as well as the actions of the catalyst during oxidation, is an important determining factor for the oxidation behaviour.
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The above findings have important implications in kinetics modelling of the coal char oxidation reaction. The existence of the compensation effect and its linear correlation will aid in modelling the char oxidation at various conversion levels. In the absence catalytic effect, from the present study, it appears that the correlation ‘ln Aapp = 0.2 Eapp + c’, with a Eapp change of 23 kJ mol1 from 10 to 80% char conversion, can be adopted for various samples (within the region encompassed by the 2 parallel dashed lines in Fig. 6), provided that the kinetic parameters for the initial char are known. In the presence of catalysts, this correlation is still valid since the overall reaction follows the intrinsic nature of carbon as aforementioned, but extra information on the extent/degree of evolution of activation energy (as results of the oxidation reaction pathway taken) would be needed, apart from the kinetic parameters for the initial char (as results of the initial char carbon structure). Therefore, exploitation of the compensation effect correlation will require more information on the catalysis during the course of oxidation, such as the amount, kind and/or form of catalysts as well as the interactions between the char carbon structure and the catalysts. 4. Conclusions In this paper, a mechanistic and kinetic study has been carried out on low-temperature oxidation of a range of carbon materials. The kinetic parameters evolve continuously throughout the oxidation conversion. Together with the direct evidence from FT-Raman spectroscopy, the results have shown the occurrence of ‘selective oxidation’ for the samples studied including graphite. Parallel to selective oxidation, the present study has also revealed the occurrence of kinetic compensation effect at various char conversion levels, for all samples, due to the heterogeneous nature and continuous evolution of char carbon structure during conversion. Under the current experimental conditions, oxidation of all samples, with or without catalysts, appears to follow the similar nature of structural condensation evolution. In the absence of catalysts, both the ‘nature’ and ‘degree/extent’ of condensation evolution of the carbon structure are similar for various carbon materials, largely dependent on the initial char carbon structure. However in the presence of catalysts, although the ‘nature’ of carbon structural condensation evolution remains similar, the ‘degree/extent’ of such evolution is also affected by the catalysis during the course of oxidation. Acknowledgements The authors gratefully acknowledge the partial support from the Cooperative Research Centre
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for Coal in Sustainable Development (CCSD) during the initial stage of the research work. Some discussion with Dong-ke Zhang is also acknowledged.
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