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Heat treatment-induced loss of combustion reactivity of a coal char: the effect of exposure to oxygen
Experimental Thermal and Fluid Science 28 (2004) 735–741 www.elsevier.com/locate/etfs
Heat treatment-induced loss of combustion reactivity of a coal char: the effect of exposure to oxygen Osvalda Senneca a
a,*
, Piero Salatino b, Sabato Masi
a
Istituto di Ricerche sulla Combustione, C.N.R., Universita’ degli Studi di Napoli Federico II, P.le Tecchio, 80125 Napoli, Italy b Dipartimento di Ingegneria Chimica, Universita’ degli Studi di Napoli Federico II, P.le Tecchio, 80125 Napoli, Italy
Abstract A South African bituminous coal was heat treated at different temperatures in the range 500–1300 C under inert conditions with or without exposure to pulses of air. The effect of the heat treatment conditions on the reactivity of the resulting char towards oxygen under chemical kinetic controlled regime was assessed. The oxygen content of chars subjected to different heat treatment conditions was also measured by elemental analysis. Results indicated that exposure to pulses of air during the early stages of heat treatment at temperatures up to 1200 C mitigates the effects of thermal annealing as far as the loss of combustion reactivity of the resulting char is concerned. Beyond about 1200 C, exposure to air is ineffective to mitigate the effects of thermal annealing on combustion reactivity. Altogether, it appears that mutual interactions exist between modifications of the carbon structure induced by thermal annealing, uptake of oxygen during heat treatment and the oxy-reactivity of the resulting char. The preliminary findings of this work would suggest that the loss of carbon combustion reactivity cannot be assessed on the basis of its time–temperature history only, but modes and extent of carbon interaction with oxygen come into play. A speculative explanation of the phenomenology is presented and discussed. 2004 Elsevier Inc. All rights reserved. Keywords: Coal; Combustion; Thermal annealing; Chemisorption
1. Introduction Coal science has significantly progressed over the last decade along the recognition of the role of the severity of heat treatment of a solid fuel on oxidation reactivity of its char. A bundle of solid-state transformations, including purely pyrolytic processes, structural reordering of the turbostratic carbon structure [1]––or thermal annealing––as well as transformations of the inorganic matter, may contribute to heat-treatment induced loss of char combustion reactivity. The nature and the kinetics of such thermally activated processes under purely inert conditions, and their impact upon char gasification reactivity, have been extensively addressed in the most recent literature. Mechanistic understanding has led to the development of models of
*
Corresponding author. Tel.: +39-0-817-682969; fax: +39-0-815936936. E-mail address: [email protected] (O. Senneca). 0894-1777/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2003.12.011
gasification kinetics which embody the effects of thermal annealing [2–8]. These models can be implemented in global reactor models for the prediction of the combustion reactivity of a char as a function of its time– temperature history inside the reactor. The hypothesis which implicitly underlies all these models is that the presence of oxygen at the carbon surface does not interfere with the course of thermal annealing. Along a different pathway, the elucidation of the detailed mechanism of the carbon–oxygen reaction has also progressed considerably by a combination of theoretical and experimental tools [9–11]. The role of the formation of oxygen surface complexes of different stability, involving ‘‘edge’’ carbon atoms or the surface of the graphene layers themselves has been highlighted, as well as the role played by surface mobility of oxygen [12,13]. Molecular orbital calculations have largely contributed to a better understanding of dissociative chemisorption of molecular oxygen and of the role of oxygen intercalation between graphitic planes [14]: intercalated oxygen is relatively stable, but it enhances gasification because a strong C–O bond weakens the
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O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
neighbor C–C bonds. Accordingly, as oxygen is dissociatively chemisorbed, a nearby oxide complex can more easily be desorbed as either CO or CO2 . It has been observed that the extent of intercalation of oxygen is limited by interlayer diffusion, and that small crystallites receive more intercalated oxygen. Molecular orbital calculations have also shown that intercalation of oxygen in the graphite layers decreases the (0 0 2) d-spacing between graphitic layers, which is to say that not only does the propensity of a coal to chemisorb oxygen depend on the carbon structure, but also the carbon structure depends on the chemisorption of oxygen. The scope of the present paper is to investigate whether and to what extent is thermal annealing is affected by parallel exposure of carbon to oxygen. The issue is relevant to establishing whether the loss of carbon combustion reactivity induced by thermal annealing in a combustor can be predicted by available thermodeactivation models on the basis of the carbon time– temperature history alone, regardless of the oxidizing versus inert nature of the environment. This goal is pursued by comparing oxy-reactivities and the oxygen contents of coal samples after different pre-treatments, which include different combinations of heat treatment under inert conditions and of pulsed exposure to oxygen during the early stages of heat treatment.
Table 1 Properties of South African coal
2. Experimental
2.3. Procedure
2.1. Materials
Samples were subjected to one of the following pretreatments:
Experiments have been carried out using a South African bituminous coal whose properties are reported in Table 1. Coal samples were ground and sieved in the size range of 75–125 lm. Heat treatment of samples was carried out in a flow of nitrogen of chromatographic grade, further purified from oxygen impurities in a copper furnace at 700 C. Air was used in combustion experiments. 2.2. Apparatus Thermogravimetric analysis of samples was carried out in a Rheometrics PL-TG1000M/1500 thermobalance purposely modified to achieve heating rates of 1000 and 10000 C/min. Approximately 2 mg of coal were loaded in the thermogravimetric analyzer for each test. An electrically heated tubular furnace with heating rate of 300 C/min was used to produce larger samples for elemental analysis. Approximately 50 mg of coal were loaded in the furnace for each test. Elemental analysis of the samples was performed in a PE-2400 CHNS/O analyser.
Net calorific value kJ kg1 Proximate analysis Volatile matter Fixed carbon Ash
26,300 23.1 61.2 15.7
Ultimate analysis Carbon Hydrogen Sulfur Nitrogen Oxygen Ash Free swelling index Random vitrinite reflectance (%)
68.0 3.8 0.6 1.2 10.7 15.7 1 0.72
Ash analysis SiO2 Al2 O3 CaO MgO K2 O Na2 O FeO MnO TiO2 P2 O5 SO3 Others
44.1 34.0 8.1 2.2 0.62 0.15 1.53 0.01 1.41 2.35 2.08 3.45
(a) heat up in nitrogen to a temperature 900 C < T < 1300 C, followed by isothermal heat treatment in nitrogen at temperature T for a time comprised between 1 and 30 min; (b) heat up in nitrogen to a temperature 900 C < T < 1300 C followed by isothermal heat treatment in nitrogen at temperature T for a time comprised between 1 and 30 min. During isothermal heat treatment pulses of air were fed to the reactor. Each air pulse was 0.4 ml and lasted 2 s in the thermobalance, 13 ml and lasted 20 s in the tubular furnace. One or more air pulses were injected during heat treatment. The first pulse was injected after 1 min of isothermal heat treatment. Additional pulses, if any, were eventually injected every other 5 min. It is worth noting that injection of a pulse of air corresponds to feeding to the thermogravimetric analyser about 4 mmol of atomic oxygen/g of carbon. If oxygen reacted entirely with the carbon present, each pulse would correspond to 2.4% or 4.8% carbon conversion degree, depending on whether CO2 or CO is generated;
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
(c) exposure to air at 530 C for 5 min, then heating up in nitrogen to 900 C followed by isothermal heat treatment in nitrogen at 900 C for 30 min; (d) heating up to 900 C in carbon dioxide followed by isothermal heat treatment at 900 C for 30 min in carbon dioxide.
737
0.140
900°C 1min 900°C 30min 1165°C 30min 1192°C 30min 1230°C 2min 1230°C 30min
0.120 0.100 0.080
Samples pretreated according to the above procedures in the thermobalance, were further reacted isothermally in the thermobalance in air at 530 C in order to assess their combustion reactivity. Samples pretreated according to the above procedures in the electrically heated furnace, were retrieved from the furnace and directly subjected to the elemental analysis. Analyses were repeated four times to check the reproducibility of results.
df/dt/(1-f), min-1
0.060 0.040 0.014 0.012 0.010 0.008 0.006 0.004 0.002
2.4. Analysis of data
0.000 0.0
The carbon conversion degree f has been calculated as f ¼ ðm0 mÞ=ðm0 mash Þ where m0 is the weight of the sample at the beginning of the isothermal combustion stage and mash the weight of the ash residue. The combustion rate during this stage was expressed as the carbon combustion rate per unit actual mass of carbon, i.e. 1=ð1 f Þðdf =dtÞ versus f . It must be noted that, due to exposure to oxygen or carbon dioxide during heat treatment, some carbon burn off might take place during pre-treatments (b), (c) and (d). This contribution was not included in the definition of f . 3. Results
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 1. Combustion rate of chars prepared under inert conditions.
different pre-treatment conditions are reported: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of one pulse of air during early heat treatment; (c) heat treatment in inert conditions for 30 min with injection of six pulses of air; (d) exposure to air at 530 C for 5 min followed by heat treatment at 900 C for 30 min; (e) heat treatment in a pure carbon dioxide atmosphere for 30 min. The comparison of data points corresponding to pre-treatments (a)–(c) indicates that
3.1. Assessment of the combustion reactivity of char 0.14 0.13
exposure to air at 530 C for 5min
0.12
exposure to CO2 at 900 C for 30mi n 6 oxygen pulses
0.11
1 oxygen pulse
0.10
No oxygen pulses
0.09 df/dt/(1-f), min-1
Fig. 1 compares the carbon combustion rates of samples heat treated under inert atmosphere for different times tHT at different temperatures THT . All the curves exhibit a common feature: a rapidly increasing rate in the early burn-off (typically less than f ¼ 0:15) followed by a relatively steady combustion rate in the range 0:15 < f < 0:8. At f > 0:8 curves relative to different samples exhibit different trends, sometimes erratic, which partly reflect the smaller reliability of data taken at f close to 1. When combustion rates of the different samples are compared with each other, it is observed that, as expected, combustion reactivity decreases with increasing severity of heat treatment conditions: a 10-fold decrease of carbon combustion rate is recorded when passing from the least annealed sample (THT ¼ 900 C, tHT ¼ 1 min) to the most annealed one (THT ¼ 1230 C, tHT ¼ 30 min). Figs. 2–4 refer to the influence of pre-treatment with oxygen and carbon dioxide. Fig. 2 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 900 C. Data points relative to five
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 2. Combustion rate of chars prepared at 900 C with different pretreatments.
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O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741 0.020 0.018 0.016
df/dt/(1-f), min-1
0.014 0.012 0.010 0.008 0.006 0.004 No oxygen pulses 1 oxygen pulse 2 oxygen pulses
0.002 0.000 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 3. Combustion rate of chars prepared at 1165 C with different pre-treatments.
0.010 0.009 0.008
df/dt/(1-f), min-1
0.007 0.006 0.005 0.004 0.003 0.002
6 oxygen pulses 1 oxygen pulse No oxygen pulses
0.001 0.000 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 4. Combustion rate of chars prepared at 1230 C with different pre-treatments.
exposure to air during heat treatment slightly increases the combustion reactivity of the resulting char. Exposure to air at 530 C for 5 min (pre-treatment (d)) affects the combustion reactivity of the resulting char even more effectively. On the other hand, exposure to pure carbon dioxide during heat treatment does not affect the reactivity of the resulting char with respect to the reactivity of the char prepared under inert conditions. Altogether results suggest that exposure of carbon to
oxygen during heat treatment mitigates the effects of thermal annealing as far as the combustion reactivity of the resulting char is concerned. Fig. 3 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 1165 C. Data points relative to three pre-treatment conditions are reported: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of 1 pulse of air prior to heat treatment; (c) heat treatment in inert conditions for 30 min with injection of two pulses of air. Data points in Fig. 3 show a fast increase of the combustion reactivity in the early burn-off. The average reactivity of samples heat treated at this temperature is lower, as expected. It can be noted that exposure of carbon samples to oxygen by injection of one or two pulses of air during heat treatment appreciably affects the combustion reactivity of the resulting char. Injection of one pulse and of two pulses of air results in an increase of the char combustion reactivity by factors of about 1.2 and 1.6, respectively, in the average. Fig. 4 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 1230 C. Pretreatment conditions considered in this case were: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of one pulse of air prior to heat treatment; (c) heat treatment in inert conditions for 30 min with injection of six pulses of air. Again, the general features of curves resembles those in Figs. 2 and 3. The nearly steady value of the combustion reactivity attained after the initial fast rise is significantly smaller than those of samples heat treated at 900 and 1165 C. The most notable feature of Fig. 4 is that data points relative to different pre-treatments overlap fairly well with each other. If one considers that as many as six pulses of air were injected during heat treatment, it can be concluded that exposure to oxygen during heat treatment at this temperature does not affect the combustion reactivity of the resulting char to any significant extent. Fig. 5 summarizes results of reactivity characterization of samples heat treated at different temperatures at a given carbon conversion degree (f ¼ 0:5). Data points reported therein correspond to either of the following pre-treatments: (a) heat treatment under inert (nitrogen) conditions for 30 min; (b) heat treatment in nitrogen for 30 min with injection of one pulse of air at the beginning of heat treatment. Data points confirm that exposure to oxygen during heat treatment enhances the combustion reactivity of the resulting char as far as the heat treatment temperature is smaller than about 1200 C. Beyond this temperature, two features can be observed: (a) the difference between reactivities of samples that were exposed to oxygen and those that were not vanishes with increasing temperature; (b) the sensitivity of thermodeactivation to temperature increases becomes apparently
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
739
and heat treated at 900 C for 30 min reduces to 0.94 mmol O/g C: most oxygen, either inherent or uptaken, has been desorbed, but still some oxygen complex is stable at the end of the pre-treatment. Some inherent oxygen is present in the sample heat treated at 1165 C, and other (0.21 mmol O/g C) is uptaken upon injection of one pulse of air. No oxygen is left upon prolonged (30 min) heat treatment at this temperature. No oxygen, either inherent or uptaken, is detected in samples prepared at the highest temperature.
0.1000
log(Rf=0.5), Rf=0.5, min-1
No oxygen pulses 1 oxygen pulse
0.0100
4. Discussion
900
950
1000
1050 1100 1150 Temperature,°C
1200
1250
Fig. 5. Combustion rates at 50% conversion of chars prepared at different temperatures.
larger. Both features should reflect a change of the relevant thermodeactivation mechanism beyond 1200 C, and this is fully consistent with previous findings [7]. 3.2. Elemental analysis: oxygen content of samples Table 2 reports the oxygen content of samples heat treated at different temperatures. Data points refer to samples that were either exposed or not to oxygen (through pulsed injection of air) during heat treatment. It can be noted that residual oxygen is found in the chars prepared under inert conditions at the mildest heat treatment conditions: 2.3% at THT ¼ 900 C, tHT ¼ 1 min, 1% at THT ¼ 1165 C, tHT ¼ 1 min, 0.5% at THT ¼ 900 C, tHT ¼ 30 min. More severe heat treatments induce the complete loss of inherent oxygen. It is interesting to analyze the uptake of oxygen associated with injection of one pulse of air at the beginning of the heat treatment stage. The samples prepared at THT ¼ 900 C and tHT ¼ 1 min uptake as much as 2.4 mmol O/g C. This is about half of the oxygen content associated with one pulse of air (ffi4 mmol O/g C). The oxygen content of the sample exposed to one pulse of air
The mechanisms and the kinetics of thermal annealing, on one hand, and the role of the formation of the oxygen complex on combustion reactivity, on the other, have been extensively addressed in the literature separately from each other. In this paper we are concerned with the mutual relationship between thermal annealing and the formation of oxygen complex, and this is apparently an unexplored subject. Results of the thermogravimetric experiments indicate that for heat treatments at temperatures up to 1200 C exposure to even small amounts of oxygen during heat treatment reduces the propensity of the carbon to undergo thermal annealing. A similar effect is obtained exposing the carbon at 530 C for 5 min prior to heat treatment. A different scenario is observed for heat treatments above 1200 C. In this case the loss of oxidation reactivity upon heat treatment becomes insensitive to the addition of oxygen pulses. On the other hand, heat treatment in a carbon dioxide atmosphere produced chars whose oxy-reactivity did not differ appreciably from that of the char prepared in nitrogen. Elemental analysis of char samples prepared with different pre-treatments suggests that exposure to even limited amounts of oxygen during heat treatment results in some oxygen uptake that is more extensive the lower the heat treatment temperature and the shorter the heat treatment time. Altogether, these findings suggest that oxygen uptake may affect the course of thermal annealing, by mitigating its effects as far as the loss of combustion reactivity is concerned. It is also interesting
Table 2 Oxygen content of samples THT (C)
900 1165 1300 a
tHT (min)
1 30 1 30 30
Heat treatment in nitrogen
Heat treatment in nitrogen + one pulse of air
O (%)
mmol O/g C (a)
O (%)
mmol O/g C (b)
2.3 0.5 1.0 0 0
1.8 0.39 0.79 0 0
5.1 1.2 1.3 0 0
4.2 0.94 1.0 0 0
Each pulse of air corresponds to an oxygen content of about 4 mmol O/g C.
Dmmol O/g Ca (b ) a) 2.4 0.55 0.21 0 0
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O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
to note that mitigation is effective at heat treatment temperatures up to about 1200 C, beyond which exposure to oxygen becomes apparently ineffective. A mechanistic explanation of these findings is lacking, and would require additional characterization of the interaction between oxygen uptake and the course of thermal annealing. Some speculative explanation can be offered at the present stage of the knowledge on the basis of the recognized role of oxygen complexes in carbon oxidation. It has been underlined, on both theoretical and experimental grounds, that at moderate temperatures molecular oxygen is dissociatively chemisorbed on the surface of graphene layers, where delocalized p electrons are available. Dissociative chemisorption on graphene layers provides an important intermediate step along the pathway of the reaction between carbon and molecular oxygen. The following interactive processes can be speculatively inferred between chemisorption on basal planes and the course of thermal annealing: • Chemisorption on basal planes is bound to be affected by the accessibility of the relevant adsorption sites, that is, by the ability of oxygen to diffuse between adjacent graphene layers to be adsorbed thereon. It is likely that this process is strongly affected by the degree of crystallinity of the turbostratic carbon structure. Under this respect thermal annealing, enhancing stacking and alignment of graphene layers, should negatively affect chemisorption on basal planes and, in turn, combustion reactivity. • Chemisorption of oxygen on graphene layers results into the intercalation of atomic oxygen in the turbostratic carbon structure. The presence of intercalated oxygen should limit the extent to which graphene layers are stacked and aligned by thermal annealing. Under this respect, the early occurrence of oxygen chemisorption on basal planes should preserve the accessibility of these planes to further adsorption, thus enhancing combustion kinetics. The fact that no enhancement of the combustion reactivity of the residual char was observed when the char was prepared by heat treatment in a carbon dioxide atmosphere is consistent with this tentative explanation. The negligible tendency of carbon dioxide to give rise to intercalation compounds with carbon rules out any possible effect on the course of thermal annealing. The finding that exposure to oxygen in the early heat treatment of samples was effective only for heat treatment temperatures up to about 1200 C would be consistent with the hypothesis that other mechanisms, different from stacking and alignment of graphene layers, might be at work beyond that temperature. This is consistent with findings recently reported by Senneca and Salatino [7] who assessed the loss of gasification
reactivity of two coals and one petroleum coke in the temperature range 900–2000 C. More insight into the interactive mechanisms of carbon oxidation and annealing are required to support the proposed speculative arguments.
5. Conclusions The effect of exposing young coal chars to oxygen during heat treatment on the rate and extent of their thermal annealing has been investigated. To this end the combustion reactivity of chars that were prepared either under inert conditions or with a limited supply of molecular oxygen were compared with each other. As far as the heat treatment temperature is smaller than about 1200 C, it is observed that chars that have been in contact with a limited amount of oxygen during heat treatment have a larger combustion reactivity than those prepared under inert conditions. Beyond 1200 C, the enhancement of combustion reactivity induced by exposure to oxygen in the heat treatment stage becomes insignificant. Elemental analysis of char samples prepared under a variety of pre-treatment conditions confirms that a nonnegligible oxygen uptake is observed upon exposure to molecular oxygen of samples at temperatures up to 1200 C. Uptake becomes insignificant beyond that temperature. Results are speculatively interpreted in the light of a mechanism according to which exposure to oxygen results into chemisorption and formation of intercalation compounds limiting the extent of graphene layers stacking and rearrangement upon heat treatment.
Acknowledgement The precious support by Ms Daniela Menghini in the experimental work is gratefully acknowledged.
References [1] J.N. Rouzaud, B. Duval, J. Leroy, in: J. Lahaye, P. Ehrburger (Eds.), Fundamental Issues in Control of Carbon Gasification Reactivity, Kluwer Academic, Dordrecht, 1991, pp. 257–265. [2] R.H. Hurt, J.-K. Sun, M. Lunden, A kinetic model of carbon burnout in pulverized coal combustion, Combust. Flame 113 (1– 2) (1998) 181–197. [3] N.V. Russell, J.R. Gibbins, C.K. Man, J. Williamson, Coal char thermal deactivation under pulverised fuel combustion conditions, Energy Fuels 14 (4) (2000) 883–888. [4] A. Zolin, A. Jensen, K. Dam-Johansen, Kinetic analysis of char thermal deactivation, Proc. Comb. Inst. 28 (2000) 2181–2188. [5] P. Salatino, O. Senneca, S. Masi, Assessment of thermodeactivation during gasification of a bituminous coal char, Energy Fuels 13 (6) (1999) 1154–1159.
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741 [6] O. Senneca, P. Russo, P. Salatino, S. Masi, The relevance of thermal annealing to the evolution of coal char gasification reactivity, Carbon 35 (1) (1997) 141–151. [7] O. Senneca, P. Salatino, Proc. of the 29th Symposium (Int.) on Combustion, vol. 29, Sapporo, 2002, pp. 485–493. [8] B. Feng, S.K. Bhatia, J.C. Barry, Structural ordering of coal char during heat treatment and its impact on reactivity, Carbon 40 (4) (2002) 481–496. [9] J.K. Floess, J.P. Longwell, A.F. Sarofim, Energy Fuels 13 (1987) 182. [10] P. Salatino, O. Senneca, S. Masi, Gasification of a coal char by oxygen and carbon dioxide, Carbon 36 (4) (1998) 443–452.
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[11] R.H. Hurt, J.M. Calo, Semi-global intrinsic kinetics for char combustion modeling, Combust. Flame 125 (3) (2001) 1138. [12] A.E. Lear, T.C. Brown, B.S. Haynes, Formation of metastable oxide complexes during the oxidation of carbon at low temperatures, Proc. Combust. Inst. 23 (1990) 1191–1197. [13] R. Schl€ ogl, G. Loose, M. Wesemann, On the mechanism of the oxidation of graphite by molecular oxygen, Solid State Ionics 43 (1990) 183. [14] Z. Pan, R.T. Yang, Strongly bonded oxygen in graphite: detection by high temperature TPD and characterization, Ind. Eng. Chem. Res. 31 (1992) 2675.
Heat treatment-induced loss of combustion reactivity of a coal char: the effect of exposure to oxygen Osvalda Senneca a
a,*
, Piero Salatino b, Sabato Masi
a
Istituto di Ricerche sulla Combustione, C.N.R., Universita’ degli Studi di Napoli Federico II, P.le Tecchio, 80125 Napoli, Italy b Dipartimento di Ingegneria Chimica, Universita’ degli Studi di Napoli Federico II, P.le Tecchio, 80125 Napoli, Italy
Abstract A South African bituminous coal was heat treated at different temperatures in the range 500–1300 C under inert conditions with or without exposure to pulses of air. The effect of the heat treatment conditions on the reactivity of the resulting char towards oxygen under chemical kinetic controlled regime was assessed. The oxygen content of chars subjected to different heat treatment conditions was also measured by elemental analysis. Results indicated that exposure to pulses of air during the early stages of heat treatment at temperatures up to 1200 C mitigates the effects of thermal annealing as far as the loss of combustion reactivity of the resulting char is concerned. Beyond about 1200 C, exposure to air is ineffective to mitigate the effects of thermal annealing on combustion reactivity. Altogether, it appears that mutual interactions exist between modifications of the carbon structure induced by thermal annealing, uptake of oxygen during heat treatment and the oxy-reactivity of the resulting char. The preliminary findings of this work would suggest that the loss of carbon combustion reactivity cannot be assessed on the basis of its time–temperature history only, but modes and extent of carbon interaction with oxygen come into play. A speculative explanation of the phenomenology is presented and discussed. 2004 Elsevier Inc. All rights reserved. Keywords: Coal; Combustion; Thermal annealing; Chemisorption
1. Introduction Coal science has significantly progressed over the last decade along the recognition of the role of the severity of heat treatment of a solid fuel on oxidation reactivity of its char. A bundle of solid-state transformations, including purely pyrolytic processes, structural reordering of the turbostratic carbon structure [1]––or thermal annealing––as well as transformations of the inorganic matter, may contribute to heat-treatment induced loss of char combustion reactivity. The nature and the kinetics of such thermally activated processes under purely inert conditions, and their impact upon char gasification reactivity, have been extensively addressed in the most recent literature. Mechanistic understanding has led to the development of models of
*
Corresponding author. Tel.: +39-0-817-682969; fax: +39-0-815936936. E-mail address: [email protected] (O. Senneca). 0894-1777/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2003.12.011
gasification kinetics which embody the effects of thermal annealing [2–8]. These models can be implemented in global reactor models for the prediction of the combustion reactivity of a char as a function of its time– temperature history inside the reactor. The hypothesis which implicitly underlies all these models is that the presence of oxygen at the carbon surface does not interfere with the course of thermal annealing. Along a different pathway, the elucidation of the detailed mechanism of the carbon–oxygen reaction has also progressed considerably by a combination of theoretical and experimental tools [9–11]. The role of the formation of oxygen surface complexes of different stability, involving ‘‘edge’’ carbon atoms or the surface of the graphene layers themselves has been highlighted, as well as the role played by surface mobility of oxygen [12,13]. Molecular orbital calculations have largely contributed to a better understanding of dissociative chemisorption of molecular oxygen and of the role of oxygen intercalation between graphitic planes [14]: intercalated oxygen is relatively stable, but it enhances gasification because a strong C–O bond weakens the
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O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
neighbor C–C bonds. Accordingly, as oxygen is dissociatively chemisorbed, a nearby oxide complex can more easily be desorbed as either CO or CO2 . It has been observed that the extent of intercalation of oxygen is limited by interlayer diffusion, and that small crystallites receive more intercalated oxygen. Molecular orbital calculations have also shown that intercalation of oxygen in the graphite layers decreases the (0 0 2) d-spacing between graphitic layers, which is to say that not only does the propensity of a coal to chemisorb oxygen depend on the carbon structure, but also the carbon structure depends on the chemisorption of oxygen. The scope of the present paper is to investigate whether and to what extent is thermal annealing is affected by parallel exposure of carbon to oxygen. The issue is relevant to establishing whether the loss of carbon combustion reactivity induced by thermal annealing in a combustor can be predicted by available thermodeactivation models on the basis of the carbon time– temperature history alone, regardless of the oxidizing versus inert nature of the environment. This goal is pursued by comparing oxy-reactivities and the oxygen contents of coal samples after different pre-treatments, which include different combinations of heat treatment under inert conditions and of pulsed exposure to oxygen during the early stages of heat treatment.
Table 1 Properties of South African coal
2. Experimental
2.3. Procedure
2.1. Materials
Samples were subjected to one of the following pretreatments:
Experiments have been carried out using a South African bituminous coal whose properties are reported in Table 1. Coal samples were ground and sieved in the size range of 75–125 lm. Heat treatment of samples was carried out in a flow of nitrogen of chromatographic grade, further purified from oxygen impurities in a copper furnace at 700 C. Air was used in combustion experiments. 2.2. Apparatus Thermogravimetric analysis of samples was carried out in a Rheometrics PL-TG1000M/1500 thermobalance purposely modified to achieve heating rates of 1000 and 10000 C/min. Approximately 2 mg of coal were loaded in the thermogravimetric analyzer for each test. An electrically heated tubular furnace with heating rate of 300 C/min was used to produce larger samples for elemental analysis. Approximately 50 mg of coal were loaded in the furnace for each test. Elemental analysis of the samples was performed in a PE-2400 CHNS/O analyser.
Net calorific value kJ kg1 Proximate analysis Volatile matter Fixed carbon Ash
26,300 23.1 61.2 15.7
Ultimate analysis Carbon Hydrogen Sulfur Nitrogen Oxygen Ash Free swelling index Random vitrinite reflectance (%)
68.0 3.8 0.6 1.2 10.7 15.7 1 0.72
Ash analysis SiO2 Al2 O3 CaO MgO K2 O Na2 O FeO MnO TiO2 P2 O5 SO3 Others
44.1 34.0 8.1 2.2 0.62 0.15 1.53 0.01 1.41 2.35 2.08 3.45
(a) heat up in nitrogen to a temperature 900 C < T < 1300 C, followed by isothermal heat treatment in nitrogen at temperature T for a time comprised between 1 and 30 min; (b) heat up in nitrogen to a temperature 900 C < T < 1300 C followed by isothermal heat treatment in nitrogen at temperature T for a time comprised between 1 and 30 min. During isothermal heat treatment pulses of air were fed to the reactor. Each air pulse was 0.4 ml and lasted 2 s in the thermobalance, 13 ml and lasted 20 s in the tubular furnace. One or more air pulses were injected during heat treatment. The first pulse was injected after 1 min of isothermal heat treatment. Additional pulses, if any, were eventually injected every other 5 min. It is worth noting that injection of a pulse of air corresponds to feeding to the thermogravimetric analyser about 4 mmol of atomic oxygen/g of carbon. If oxygen reacted entirely with the carbon present, each pulse would correspond to 2.4% or 4.8% carbon conversion degree, depending on whether CO2 or CO is generated;
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
(c) exposure to air at 530 C for 5 min, then heating up in nitrogen to 900 C followed by isothermal heat treatment in nitrogen at 900 C for 30 min; (d) heating up to 900 C in carbon dioxide followed by isothermal heat treatment at 900 C for 30 min in carbon dioxide.
737
0.140
900°C 1min 900°C 30min 1165°C 30min 1192°C 30min 1230°C 2min 1230°C 30min
0.120 0.100 0.080
Samples pretreated according to the above procedures in the thermobalance, were further reacted isothermally in the thermobalance in air at 530 C in order to assess their combustion reactivity. Samples pretreated according to the above procedures in the electrically heated furnace, were retrieved from the furnace and directly subjected to the elemental analysis. Analyses were repeated four times to check the reproducibility of results.
df/dt/(1-f), min-1
0.060 0.040 0.014 0.012 0.010 0.008 0.006 0.004 0.002
2.4. Analysis of data
0.000 0.0
The carbon conversion degree f has been calculated as f ¼ ðm0 mÞ=ðm0 mash Þ where m0 is the weight of the sample at the beginning of the isothermal combustion stage and mash the weight of the ash residue. The combustion rate during this stage was expressed as the carbon combustion rate per unit actual mass of carbon, i.e. 1=ð1 f Þðdf =dtÞ versus f . It must be noted that, due to exposure to oxygen or carbon dioxide during heat treatment, some carbon burn off might take place during pre-treatments (b), (c) and (d). This contribution was not included in the definition of f . 3. Results
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 1. Combustion rate of chars prepared under inert conditions.
different pre-treatment conditions are reported: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of one pulse of air during early heat treatment; (c) heat treatment in inert conditions for 30 min with injection of six pulses of air; (d) exposure to air at 530 C for 5 min followed by heat treatment at 900 C for 30 min; (e) heat treatment in a pure carbon dioxide atmosphere for 30 min. The comparison of data points corresponding to pre-treatments (a)–(c) indicates that
3.1. Assessment of the combustion reactivity of char 0.14 0.13
exposure to air at 530 C for 5min
0.12
exposure to CO2 at 900 C for 30mi n 6 oxygen pulses
0.11
1 oxygen pulse
0.10
No oxygen pulses
0.09 df/dt/(1-f), min-1
Fig. 1 compares the carbon combustion rates of samples heat treated under inert atmosphere for different times tHT at different temperatures THT . All the curves exhibit a common feature: a rapidly increasing rate in the early burn-off (typically less than f ¼ 0:15) followed by a relatively steady combustion rate in the range 0:15 < f < 0:8. At f > 0:8 curves relative to different samples exhibit different trends, sometimes erratic, which partly reflect the smaller reliability of data taken at f close to 1. When combustion rates of the different samples are compared with each other, it is observed that, as expected, combustion reactivity decreases with increasing severity of heat treatment conditions: a 10-fold decrease of carbon combustion rate is recorded when passing from the least annealed sample (THT ¼ 900 C, tHT ¼ 1 min) to the most annealed one (THT ¼ 1230 C, tHT ¼ 30 min). Figs. 2–4 refer to the influence of pre-treatment with oxygen and carbon dioxide. Fig. 2 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 900 C. Data points relative to five
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 2. Combustion rate of chars prepared at 900 C with different pretreatments.
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O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741 0.020 0.018 0.016
df/dt/(1-f), min-1
0.014 0.012 0.010 0.008 0.006 0.004 No oxygen pulses 1 oxygen pulse 2 oxygen pulses
0.002 0.000 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 3. Combustion rate of chars prepared at 1165 C with different pre-treatments.
0.010 0.009 0.008
df/dt/(1-f), min-1
0.007 0.006 0.005 0.004 0.003 0.002
6 oxygen pulses 1 oxygen pulse No oxygen pulses
0.001 0.000 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Carbon conversion degree, f
Fig. 4. Combustion rate of chars prepared at 1230 C with different pre-treatments.
exposure to air during heat treatment slightly increases the combustion reactivity of the resulting char. Exposure to air at 530 C for 5 min (pre-treatment (d)) affects the combustion reactivity of the resulting char even more effectively. On the other hand, exposure to pure carbon dioxide during heat treatment does not affect the reactivity of the resulting char with respect to the reactivity of the char prepared under inert conditions. Altogether results suggest that exposure of carbon to
oxygen during heat treatment mitigates the effects of thermal annealing as far as the combustion reactivity of the resulting char is concerned. Fig. 3 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 1165 C. Data points relative to three pre-treatment conditions are reported: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of 1 pulse of air prior to heat treatment; (c) heat treatment in inert conditions for 30 min with injection of two pulses of air. Data points in Fig. 3 show a fast increase of the combustion reactivity in the early burn-off. The average reactivity of samples heat treated at this temperature is lower, as expected. It can be noted that exposure of carbon samples to oxygen by injection of one or two pulses of air during heat treatment appreciably affects the combustion reactivity of the resulting char. Injection of one pulse and of two pulses of air results in an increase of the char combustion reactivity by factors of about 1.2 and 1.6, respectively, in the average. Fig. 4 reports carbon combustion rates versus carbon burn-off for samples heat treated at THT ¼ 1230 C. Pretreatment conditions considered in this case were: (a) heat treatment in inert (nitrogen) atmosphere for 30 min; (b) heat treatment in inert atmosphere for 30 min with injection of one pulse of air prior to heat treatment; (c) heat treatment in inert conditions for 30 min with injection of six pulses of air. Again, the general features of curves resembles those in Figs. 2 and 3. The nearly steady value of the combustion reactivity attained after the initial fast rise is significantly smaller than those of samples heat treated at 900 and 1165 C. The most notable feature of Fig. 4 is that data points relative to different pre-treatments overlap fairly well with each other. If one considers that as many as six pulses of air were injected during heat treatment, it can be concluded that exposure to oxygen during heat treatment at this temperature does not affect the combustion reactivity of the resulting char to any significant extent. Fig. 5 summarizes results of reactivity characterization of samples heat treated at different temperatures at a given carbon conversion degree (f ¼ 0:5). Data points reported therein correspond to either of the following pre-treatments: (a) heat treatment under inert (nitrogen) conditions for 30 min; (b) heat treatment in nitrogen for 30 min with injection of one pulse of air at the beginning of heat treatment. Data points confirm that exposure to oxygen during heat treatment enhances the combustion reactivity of the resulting char as far as the heat treatment temperature is smaller than about 1200 C. Beyond this temperature, two features can be observed: (a) the difference between reactivities of samples that were exposed to oxygen and those that were not vanishes with increasing temperature; (b) the sensitivity of thermodeactivation to temperature increases becomes apparently
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
739
and heat treated at 900 C for 30 min reduces to 0.94 mmol O/g C: most oxygen, either inherent or uptaken, has been desorbed, but still some oxygen complex is stable at the end of the pre-treatment. Some inherent oxygen is present in the sample heat treated at 1165 C, and other (0.21 mmol O/g C) is uptaken upon injection of one pulse of air. No oxygen is left upon prolonged (30 min) heat treatment at this temperature. No oxygen, either inherent or uptaken, is detected in samples prepared at the highest temperature.
0.1000
log(Rf=0.5), Rf=0.5, min-1
No oxygen pulses 1 oxygen pulse
0.0100
4. Discussion
900
950
1000
1050 1100 1150 Temperature,°C
1200
1250
Fig. 5. Combustion rates at 50% conversion of chars prepared at different temperatures.
larger. Both features should reflect a change of the relevant thermodeactivation mechanism beyond 1200 C, and this is fully consistent with previous findings [7]. 3.2. Elemental analysis: oxygen content of samples Table 2 reports the oxygen content of samples heat treated at different temperatures. Data points refer to samples that were either exposed or not to oxygen (through pulsed injection of air) during heat treatment. It can be noted that residual oxygen is found in the chars prepared under inert conditions at the mildest heat treatment conditions: 2.3% at THT ¼ 900 C, tHT ¼ 1 min, 1% at THT ¼ 1165 C, tHT ¼ 1 min, 0.5% at THT ¼ 900 C, tHT ¼ 30 min. More severe heat treatments induce the complete loss of inherent oxygen. It is interesting to analyze the uptake of oxygen associated with injection of one pulse of air at the beginning of the heat treatment stage. The samples prepared at THT ¼ 900 C and tHT ¼ 1 min uptake as much as 2.4 mmol O/g C. This is about half of the oxygen content associated with one pulse of air (ffi4 mmol O/g C). The oxygen content of the sample exposed to one pulse of air
The mechanisms and the kinetics of thermal annealing, on one hand, and the role of the formation of the oxygen complex on combustion reactivity, on the other, have been extensively addressed in the literature separately from each other. In this paper we are concerned with the mutual relationship between thermal annealing and the formation of oxygen complex, and this is apparently an unexplored subject. Results of the thermogravimetric experiments indicate that for heat treatments at temperatures up to 1200 C exposure to even small amounts of oxygen during heat treatment reduces the propensity of the carbon to undergo thermal annealing. A similar effect is obtained exposing the carbon at 530 C for 5 min prior to heat treatment. A different scenario is observed for heat treatments above 1200 C. In this case the loss of oxidation reactivity upon heat treatment becomes insensitive to the addition of oxygen pulses. On the other hand, heat treatment in a carbon dioxide atmosphere produced chars whose oxy-reactivity did not differ appreciably from that of the char prepared in nitrogen. Elemental analysis of char samples prepared with different pre-treatments suggests that exposure to even limited amounts of oxygen during heat treatment results in some oxygen uptake that is more extensive the lower the heat treatment temperature and the shorter the heat treatment time. Altogether, these findings suggest that oxygen uptake may affect the course of thermal annealing, by mitigating its effects as far as the loss of combustion reactivity is concerned. It is also interesting
Table 2 Oxygen content of samples THT (C)
900 1165 1300 a
tHT (min)
1 30 1 30 30
Heat treatment in nitrogen
Heat treatment in nitrogen + one pulse of air
O (%)
mmol O/g C (a)
O (%)
mmol O/g C (b)
2.3 0.5 1.0 0 0
1.8 0.39 0.79 0 0
5.1 1.2 1.3 0 0
4.2 0.94 1.0 0 0
Each pulse of air corresponds to an oxygen content of about 4 mmol O/g C.
Dmmol O/g Ca (b ) a) 2.4 0.55 0.21 0 0
740
O. Senneca et al. / Experimental Thermal and Fluid Science 28 (2004) 735–741
to note that mitigation is effective at heat treatment temperatures up to about 1200 C, beyond which exposure to oxygen becomes apparently ineffective. A mechanistic explanation of these findings is lacking, and would require additional characterization of the interaction between oxygen uptake and the course of thermal annealing. Some speculative explanation can be offered at the present stage of the knowledge on the basis of the recognized role of oxygen complexes in carbon oxidation. It has been underlined, on both theoretical and experimental grounds, that at moderate temperatures molecular oxygen is dissociatively chemisorbed on the surface of graphene layers, where delocalized p electrons are available. Dissociative chemisorption on graphene layers provides an important intermediate step along the pathway of the reaction between carbon and molecular oxygen. The following interactive processes can be speculatively inferred between chemisorption on basal planes and the course of thermal annealing: • Chemisorption on basal planes is bound to be affected by the accessibility of the relevant adsorption sites, that is, by the ability of oxygen to diffuse between adjacent graphene layers to be adsorbed thereon. It is likely that this process is strongly affected by the degree of crystallinity of the turbostratic carbon structure. Under this respect thermal annealing, enhancing stacking and alignment of graphene layers, should negatively affect chemisorption on basal planes and, in turn, combustion reactivity. • Chemisorption of oxygen on graphene layers results into the intercalation of atomic oxygen in the turbostratic carbon structure. The presence of intercalated oxygen should limit the extent to which graphene layers are stacked and aligned by thermal annealing. Under this respect, the early occurrence of oxygen chemisorption on basal planes should preserve the accessibility of these planes to further adsorption, thus enhancing combustion kinetics. The fact that no enhancement of the combustion reactivity of the residual char was observed when the char was prepared by heat treatment in a carbon dioxide atmosphere is consistent with this tentative explanation. The negligible tendency of carbon dioxide to give rise to intercalation compounds with carbon rules out any possible effect on the course of thermal annealing. The finding that exposure to oxygen in the early heat treatment of samples was effective only for heat treatment temperatures up to about 1200 C would be consistent with the hypothesis that other mechanisms, different from stacking and alignment of graphene layers, might be at work beyond that temperature. This is consistent with findings recently reported by Senneca and Salatino [7] who assessed the loss of gasification
reactivity of two coals and one petroleum coke in the temperature range 900–2000 C. More insight into the interactive mechanisms of carbon oxidation and annealing are required to support the proposed speculative arguments.
5. Conclusions The effect of exposing young coal chars to oxygen during heat treatment on the rate and extent of their thermal annealing has been investigated. To this end the combustion reactivity of chars that were prepared either under inert conditions or with a limited supply of molecular oxygen were compared with each other. As far as the heat treatment temperature is smaller than about 1200 C, it is observed that chars that have been in contact with a limited amount of oxygen during heat treatment have a larger combustion reactivity than those prepared under inert conditions. Beyond 1200 C, the enhancement of combustion reactivity induced by exposure to oxygen in the heat treatment stage becomes insignificant. Elemental analysis of char samples prepared under a variety of pre-treatment conditions confirms that a nonnegligible oxygen uptake is observed upon exposure to molecular oxygen of samples at temperatures up to 1200 C. Uptake becomes insignificant beyond that temperature. Results are speculatively interpreted in the light of a mechanism according to which exposure to oxygen results into chemisorption and formation of intercalation compounds limiting the extent of graphene layers stacking and rearrangement upon heat treatment.
Acknowledgement The precious support by Ms Daniela Menghini in the experimental work is gratefully acknowledged.
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