- Email: [email protected]
Modelling of volatile product evolution in coal pyrolysis. The role of aerial oxidation
Journal of Analytical and Applied Pyrolysis 44 (1998) 205 – 218
Modelling of volatile product evolution in coal pyrolysis. The role of aerial oxidation G. de la Puente, G. Marba´n, E. Fuente, J.J. Pis * Instituto Nacional del Carbo´n, CSIC. Apartado 73, 33080 O6iedo, Spain Received 14 September 1997; accepted 22 September 1997
Abstract Non-isothermal thermogravimetry has been applied to study the pyrolysis behaviour of coal and its changes as a consequence of aerial oxidation. A mathematical model has been developed to calculate the kinetic parameters of the coal pyrolysis process, which takes into account three different groups of thermal decomposition reactions for fresh coal, and two groups for oxidised coals. The agreement between the model predictions, for any heating rate, and the experimental values was fairly good. The volatile matter release profile of fresh coal can be described using three stages: Stage I (250 – 475°C), mainly light species are liberated; Stage II (475-575°C), characteristic of bituminous coals, high molecular weight species (tar) and hydrocarbons (primary gases) are evolved, which may lead to melting (metaplast); Stage III ( \575°C), secondary gases are produced while undergoing ring condensation and leading to the formation of coke. The decrease in the amount of aliphatic hydrocarbons released in the pyrolysis of oxidised coals implies that stage II is not observed, ‘mobile’ phase is insufficient to allow aromatic planar units to slide over each other, the melting cannot begin, and oxidised coals pass directly to stage III. This causes loss of plasticity and is therefore, responsible for the degradation of coking properties, as a consequence of oxidation. The aromatic fractions, that cannot rearrange, form a poor ordered structure and yield a powdered char. © 1998 Elsevier Science B.V. Keywords: Coal oxidation; Pyrolysis modelling; Reaction kinetics; Thermogravimetry; Differential thermal analysis
* Corresponding author. Tel.: +34 8 5280800; fax: + 34 8 5297662. 0165-2370/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 5 - 2 3 7 0 ( 9 7 ) 0 0 0 7 8 - 8
206
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
1. Introduction Coal devolatilisation is an intrinsic step in most coal conversion processes, whether it is gasification, combustion or carbonisation. In the course of the carbonisation process, as temperature increases important changes in structure and physical conditions of the solid phase occurs, resulting in the formation of the solid residue of carbonisation: coke or char. The application of the thermal analysis method to study the combustion and pyrolysis behaviour of fossil fuels has gained a wide acceptance among research workers, which is of exceptional significance for industry and the economy [1]. Elder and Harris [2] investigated the thermal characteristics of Kentucky bituminous coals undergoing pyrolysis in an inert atmosphere. The exothermic heat flow from 300 – 550°C, where the major weight loss occurs, has been associated with the primary carbonisation process and the development of the plastic state. The end of this stage is the onset of secondary gasification, which is responsible for coke formation. Mathematical models of hydrocarbon formation are based on first-order kinetics conforming to the Arrhenius equation. In such models [3,4], ‘n’ parallel and independent reactions were used to explain the formation of different products during the primary cracking in pyrolysis. With respect to secondary pyrolysis, Tyler [5] studied the devolatilisation of bituminous coals in a fluidized bed reactor in the temperature range between 500 and 900°C, Doolan et al. [6] studied the cracking reactions of tars generated in the flash pyrolysis of coal at 600 – 1700°C, and Serio et al. [7] studied the yield and kinetics of tars generated by several types of coal at 500–900°C. Coal oxidation occurs when coal is exposed to the air during the operations of extraction, preparation, transport and storage. Oxidation produces remarkable changes in coal properties [8 – 11], some of which have a negative effect in the industrial use of coal. For these reasons, the study of aerial oxidation of coals has received much attention in literature, the detrimental effect that oxidation has upon the coking and caking properties, through the loss of plastic properties, is well documented [12 – 15]. Changes in coal behaviour are mainly due to the chemical and structural modifications taking place during air oxidation. The objective of the present work is to develop a general model of coal pyrolysis that predicts the behaviour of coal and its changes during thermal treatment as a consequence of pre-oxidation. It is necessary to know the amount and evolution rate of gas species as a function of temperature. A knowledge of thermal reaction kinetics is necessary to understand and predict the pyrolysis behaviour of coals. A multistep pyrolysis model has been developed which takes into account three different groups of thermal decomposition reactions for fresh coal, whilst the pyrolysis of oxidised coals is described by means of just two reactions.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
207
2. Experimental A medium volatile bituminous coal, Amonate, was used in this work. The coal was dried under atmospheric pressure at 40°C, and then it was ground to a particle size below 150 mm. Representative coal samples were obtained using a small sample divider (riffle). Oxidation was carried out under air flow in a forced convection furnace at 200°C and these conditions were maintained for times varying from 6 h to 14 days in order to prepare samples with various oxidation degrees. The samples were uniformly distributed in trays in a thin layer, in order to have a similar air exposition for the particles. After oxidation, coal samples were stored under argon to avoid ulterior oxidation. Chemical characterisation (proximate and ultimate analyses) of fresh coal and oxidised samples was made following international standard procedures. The elemental analysis was carried out using LECO CHN 600 and LECO SC 132 equipment, and the oxygen content was calculated by the difference. Results are presented in Table 1. Differential thermal analysis was used to determine the pyrolysis behaviour of fresh and oxidised coals. Experiments were carried out in a Setaram TAG 24 thermal analysis system. Coal samples of 25 mg, nitrogen flow rate of 50 ml min − 1 and a linear heating rate of 50°C min − 1 were used. Additionally, some experiments were performed at different heating rates (25 and 75°C min − 1). The weight loss (thermogravimetric TG signal) and the rate of weight loss (differential thermogravimetric DTG signal) as a function of time or temperature were recorded while the coals where subjected to a controlled temperature program. Caking properties were determined using the free-swelling index standard test method ASTM D720-91 [16].
Table 1 Chemical characterisation of fresh coal and oxidised samples Oxidation time (days)
0 0.25 3 5 7 10 14 a b
Elemental compositiona (%)
Proximate analysis
Moisture (%)
Volatile mattera (%)
Ashb (%)
C
H
N
S
Odiff
0.5 1.8 1.2 1.8 2.6 2.7 1.2
25.1 23.2 27.0 29.7 32.0 32.4 33.4
6.0 8.2 6.9 8.0 6.6 6.7 6.7
90.3 82.2 79.6 78.0 76.5 76.2 75.6
4.9 2.7 2.6 2.2 2.2 2.2 2.0
1.4 1.5 1.3 1.4 1.2 1.3 1.2
0.8 0.7 0.7 0.7 0.7 0.7 0.6
2.6 12.9 15.8 17.7 19.4 19.7 20.6
Dry ash free basis (daf). Dry basis.
208
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
3. Results and discussion The rate of weight loss as a function of temperature for the parent coal is shown in Fig. 1. For fresh coal three temperature regions can be observed, which correspond to three groups of thermal decomposition reactions. After the loss of moisture, around 250°C the desorption of gases started. At about 525°C the rate of thermal decomposition reached a maximum. A subsequent final phase of degasification was observed. These results are in good agreement with those of other authors [17]. The first region corresponds to the loss of light molecules which are linked physically to the carbonaceous matrix. At higher temperatures the breaking of thermal decomposition bonds occurs, originating the formation of tar and hydrocarbons. Finally, at higher temperatures the pyrolysis is predominantly characterised by the formation of hydrogen. In the case of bituminous coals, they can melt during the plastic range (350– 500°C). Afterwards the conversion of semicoke to high-temperature coke, characterised by the formation of a more or less regular structure at temperatures above 700°C takes place. From the shape of the thermogravimetric curve DTG (Fig. 1) the presence of three stages in the pyrolysis of fresh coal can be deduced: Stage I, between 250 and 475°C, during which the weight loss is small (B 0.25% s − 1). Stage II, between 475 and 575°C, involving a very important weight loss for the fresh bituminous coal. Stage III, above 575°C, this implies a relatively small weight loss (B 0.25% s − 1).
Fig. 1. Rate of weight loss in the pyrolysis of fresh coal.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
209
Fig. 2. Rate of weight loss in the pyrolysis of the coals oxidised for various periods of time (from 0.25 to 14 days).
When the oxidised coals were considered (Fig. 2), two peaks were observed. The most important change in DTG curves as a consequence of coal oxidation was the drastic decrease in the rate of weight loss around 500°C. At this temperature, where fresh coal presents the maximum, the oxidised coals show an inflexion point. This result indicates that stage 2 of pyrolysis has practically disappeared for oxidised coals. From the shape of these thermogravimetric curves (DTG) the presence of only two stages in the pyrolysis of oxidised coals can be inferred: Stage I. The weight loss is more important than for fresh coal, and it increases as the oxidation degree increases. After 7 days of treatment almost no changes were observed. Stage III. The oxidised samples show a much more important volatile release at this stage, but an increase in the oxidation time causes a decrease in the relative quantity of volatile matter evolved at these temperatures.
3.1. Kinetics of thermal decomposition The kinetic parameters of the thermal decomposition reactions were calculated using a differential method assuming that: (i) various parallel reactions exist [18]; (ii) the kinetic equations of each reaction are of first order [19–21]; and the rate constants follow Arrhenius’ law. The model proposed in this work considers that the components of coal can evolve as non-competitive independent reactions [22]. The following expression can therefore be established:
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
210
dXi =ki (1 − Xi ) dt
(1)
where i indicates reactions 1, 2 or 3, and Xi is the fraction of volatile matter released during reaction i: Xi =
mti −mi mti
(2)
where mti is the total weight of volatiles that evolved during reaction i, and mi is the weight of the remaining volatiles that are released in reaction i at time t and temperature T. The kinetic constant ki can be expressed, according to Arrhenius’ law: ki = A i e
−
Ei RT
(3)
where Ei and Ai represent the activation energy and the pre-exponential factor for the reaction i. This model assumes that during each reaction i a fraction fi of the total volatile matter is released. The mathematical model is based on the calculus of the parameters fi, Ei and Ai that give the same weight as the experimental data obtained in the thermobalance. At a time t the fraction Xi was calculated from:
&
Xi
Xij
dXi = Ai (1 − Xi )
&
t
e
−
Ei RT
dt
(4)
tj
&
where Xij is the fraction Xi at a time tj lower than t. Xi =1 − exp ln(1 −Xij ) −
t
Ai e
−
Ei RT
dt
(5)
tj
With the obtained values of Xi the weight loss at a time t was calculated: n
m = mo − % Xi fi mt
(6)
i=1
where m0 is the initial weight of sample and mt the total weight evolved during the pyrolysis process, data obtained in the thermogravimetric analyser. The values of weight as a function of time, obtained from the model were compared with experimental data (TG curve). The rate of weight loss was calculated: −
n E dm − i = mt % fi (1 − Xi )Ai e RT dt i=1
(7)
and the results obtained from the multistep kinetic model were tested by comparing the experimental and simulated DTG curves. Taking into account the above discussed shape of thermogravimetric curves (Figs. 1 and 2), it was assumed that volatile matter was released in three parallel reactions for fresh coal, and two reactions for coals oxidised at various degrees.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
211
Fig. 3. Comparison between experimental data and theoretical values predicted using the kinetic model for fresh coal.
An illustration of the experimental rate of weight loss, − dm/dt, and the curves obtained from the application of the kinetic model for the global process and three independent reactions considered for fresh coal, is given in Fig. 3. The agreement between the predicted and experimental data is quite good. The kinetic parameters obtained are presented in Table 2, where Tmi represents the temperature of maximum weight loss during reaction i. It is observed that during reactions 2 and 3 most of the volatile compounds (95%) contained in fresh coal were liberated. Due to the great complexity of the reactions taking place during pyrolysis it is not possible, without further analysis, to determine from the obtained kinetic data which compounds are produced in each reaction. Based on the similarity of activation energy and Arrhenius factor values obtained for reaction 2 and those reported by Suuberg [23] for the formation of light aliphatic hydrocarbons during the pyrolysis of bituminous coal, the results obtained from this model indicate that this kind of compounds are produced at temperatures of 475–575°C. Table 2 Kinetic parameters obtained for fresh coal
Tmi (°C) Ei (kcal mol−1) Ai (s−1) fi (%)
Reaction 1
Reaction 2
Reaction 3
394 25.7 6.33×106 4.4
524 46.4 1.62×1011 50.7
650 10.5 1.64 44.9
212
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
Fig. 4. Comparison between experimental data and theoretical values predicted using the kinetic model for coal oxidised at 200°C for 7 days.
Fig. 4 gives a general comparison of volatile release profiles for oxidised coals, the experimental rate of weight loss, − dm/dt, and the curves obtained from the application of the kinetic model for the global process and the two independent reactions considered for the coal oxidised during 7 days at 200°C. In all cases a very good agreement was found between experimental and predicted data. The obtained kinetic parameters are given in Table 3. No significant differences were observed, indicating that pyrolysis takes place using a similar pathway for coal oxidised at various degrees. The amount of volatile matter released during reaction 1 increased as the oxidation degree increased. Both the kinetic parameters and the temperature of the maximum weight loss, Tm3, of reaction 3 for fresh coal are very similar to those of reaction 3 for the oxidised coals, the difference being the yield increased from 45% for fresh coal to 90% for the oxidised coals. Reaction 2 corresponds to fresh coal, with a maximum weight loss at 524°C, this does not appear for the pyrolysis of oxidised coals. The low energy produced in the combustion of volatile matter of oxidised coals [24] compared to that of fresh coals confirms that fresh coal liberates light hydrocarbons (reaction 2) with a higher heating value than volatile matter produced from oxidised coals, with a higher CO2 and H2O content. It has been reported that pre-oxidation at low temperatures (B 300°C) in air can reduce or even destroy the thermoplastic properties of coals [25–27]. Caking properties were determined using the free-swelling index standard test method. The fresh sample presented a free swelling index of 8, and swelling disappeared after 6 h of oxidation, giving a free swelling index of 0 for the rest of the series. According
Tm1 (°C) 372 431 424 434 418 429
tox (days) 0.25 1 5 7 10 14
15.2 16.8 19.1 16.0 16.4 18.5
E1 (kcal mol−1)
Table 3 Kinetic parameters obtained for oxidised coals
2.33 2.50 1.67 1.17 2.30 3.03
A1×10−3 (s−1) 2.1 3.9 8.4 8.3 8.7 8.8
f1 (%) 652 638 645 620 622 626
Tm3 (°C)
11.4 11.0 11.8 10.3 10.2 10.3
E3 (kcal mol−1)
2.73 2.50 3.95 1.81 1.72 1.82
A3×10−3 (s−1)
97.9 96.1 91.6 91.7 91.3 91.2
f3 (%)
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218 213
214
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
Table 4 Kinetic parameters calculated for coal pyrolysed at different heating rates (tox =10 days) l (°C min−1)
E1 (kcal mol−1)
A1 · 10−3 (s−1)
f1 (%)
E3 (kcal mol−1)
A3×10−3 (s−1)
f3 (%)
25 50 75
16.4 16.4 16.4
1.15 2.30 3.45
8.7 8.7 8.7
10.2 10.2 10.2
0.86 1.72 2.58
91.3 91.3 91.3
to this, the results are in agreement with the hypothesis [28] that the reduction of the main pyrolysis peak in the DTG curve is proportional to the degradation of plastic properties. Based on the absence of reaction 2, corresponding to the formation of light aliphatic hydrocarbons from bituminous coals, it seems reasonable to assume that the kind of volatile products in the oxidised coals, almost free of light aliphatic hydrocarbons, is the cause of the loss of plasticity and, is therefore responsible for the degradation of coking properties of coals as a consequence of oxidation. The proposed model gives details on the different behaviour of the coals during pyrolysis. The validation of this model was based on its ability to predict the volatile release profiles at different heating rates than that used for its proposition. Experiments were carried out for coal oxidised during 10 days at heating rates, l, of 25, 50 and 75°C min − 1. The kinetic parameters were calculated using the proposed model, and the results are given in Table 4. In Fig. 5 the rates of volatile
Fig. 5. Volatile release profiles at different heating rates, comparison between predicted values (lines) and experimental data (symbols).
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
215
release are plotted against temperature for each heating rate, and the predicted data are compared with the experimental ones. For all heating rates the same values were obtained both for the activation energies, Ei, and for the relative fractions evolved, fi, corroborating the applicability of the model. A linear dependency of the Arrhenius factor, Ai, with the heating rate, l, is observed and the temperatures of maximum rate of weight loss, Tmi, are independent of the heating rate.
3.2. Macroscopic pyrolysis model According to the obtained results, coal pyrolysis can be described in terms of the stages shown in Fig. 6. This view is an extension of a model originally proposed by Chermin and van Krevelen [29], modified by Serio [30] and adapted in this work. When a fresh bituminous coal is considered (continuous lines and gross characters in Fig. 6): During stage I some light species, that exist as guest molecules or are formed by breaking of very weak bonds, are released (light species). During stage II, characteristic of bituminous coals, further bond-breaking and reduction of hydrogen bonding occurs, leading to evolution of high molecular weight species (tar) and hydrocarbons (primary gases), and lead to melting (metaplast). During stage III, the products can continue to react; the metaplast can evolve secondary gases, mainly CO and H2, while undergoing ring condensation and leading to the formation of coke. In the case of oxidised bituminous coals (dotted lines and underlined characters in Fig. 6): In stage I, (Figs. 1 and 2) an increase in the proportion of volatile products released at temperatures around 400°C was observed. The weight loss at these temperatures is attributed to the diffusion of molecules with a low molecular weight (B150–200) through the porous network of the coal [31]. Oxidation creates functional groups in the structure of the coal that can be more easily broken than
Fig. 6. Different stages of the pyrolysis process for fresh (solid lines and gross characters) and oxidised bituminous coals (dotted lines and underlined characters).
216
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
those of fresh coal. This observation is in line with the example that C–H bond strength is 103 kcal mol − 1 compared with 83, 78 and 91 kcal mol − 1 for the C–O, R –O –R and R – OH bonds, respectively. The presence of these functional groups in oxidised coals facilitates greater decomposition at low temperatures, that will be released mainly as CO and CO2. Taking into account that aliphatic CH groups have been reported as the key factor in the development of fluidity [32], the big decrease of this functionality observed in the FT-IR spectrum of the Amonate coal oxidised at 200°C [33] can explain the destruction of the plastic properties of this coal. Stage II concludes when it has consumed all the available hydrogen from the aliphatic and hydroaromatic fraction of the coal to saturate the free radicals produced. The big decrease in the amount of aliphatic hydrocarbons produced means that reaction 2 is not observed, melting can not begin, and oxidised coals pass directly to stage III. At stage III, ring condensation begins above 575°C, with the elimination of hydrogen. At high temperatures, CO is produced from the cracking of oxygenated functions. The aromatic fractions, in the case of oxidised coals are not bound, these give rise to the formation of a powdered char. The development of fluidity in coal is still poorly understood. It has been suggested that both the number and the nature of cross-links in the coal are key factors in the process. To illustrate the effect of oxidation, a macroscopic view of coal plasticity is presented in Fig. 7, based on one given by Spiro [34] and modified in this work. On the left side the stages for fresh coal are presented, whilst the right side gives an outline of the stages oxidised coal undergoes. According to this
Fig. 7. Macroscopic view of coal plasticity.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
217
macroscopic view, a bituminous coal can be described as a network of macromolecules with trapped molecules (Fig. 7A). Many authors distinguish a ‘mobile’ and a ‘immobile’ phase in coal [35,36]. The mobile phase consists of trapped molecules and fragments of the network. Upon heating, the macromolecules decompose and the aromatic planar units can slide over each other, in a more or less free way (Fig. 7B). This is a quite liquid state, in which the coal exhibits viscoelastic properties. The molecules, which are initially trapped, are free now and can act as lubricants. Concurrently, thermal decomposition at progressively higher temperatures leads to evolution of gases and tars. These internally generated volatile components are trapped within the viscous mass and cause swelling. At a particular temperature the expanded mass solidifies (Fig. 7C), due to the escape of gaseous molecules, the lack of lubricants, and the occurrence of condensation reactions, gives rise to the formation of coke. The volatile matter released from oxidised coals, almost free of light aliphatic hydrocarbons, as discussed above on the disappearance of stage II of pyrolysis, means that not enough ‘mobile’ phase is formed to allow aromatic planar units to slide over each other due to a lack of lubricants, and is the cause of the destruction of the thermoplastic properties of coals. The aromatic fractions, that in the case of oxidised coals cannot rearrange, form a poorly ordered structure (Fig. 7D) and yield a powdered char.
4. Conclusions From the shape of the thermogravimetric curve (DTG) the presence of three stages in the pyrolysis of fresh bituminous coal can be deduced. The most important change in DTG curves as a consequence of coal oxidation was the drastic decrease in the rate of weight loss around 500°C. From the shape of these thermogravimetric curves (DTG) the presence of only two stages in the pyrolysis of oxidised bituminous coals can be inferred. The kinetic parameters of thermal decomposition reactions were calculated using a differential method considering that the components of coal can evolve due to non-competitive independent reactions. The model was tested using experimental data and the agreement found was fairly good. Stage II, corresponds to the formation of light aliphatic hydrocarbons from bituminous coals, this does not appear for the pyrolysis of oxidised coals. To illustrate the effect of oxidation, a macroscopic view of coal plasticity is presented. A bituminous coal can be described as a network of macromolecules with trapped molecules in it. Upon heating the macromolecules decompose and the aromatic planar units can slide over each other, more or less free. This is a quite liquid state, in which the coal exhibits viscoelastic properties. The decrease in the amount of aliphatic hydrocarbons released in the pyrolysis of oxidised coals means that stage II is not observed, not enough ‘mobile’ phase is formed to allow aromatic planar units to slide over each other, the melting cannot begin, and oxidised coals pass directly to stage III. This causes the loss of plasticity and, therefore is responsible for the degradation of coking properties of coals as a consequence of oxidation.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
218
Acknowledgements The authors wish to thank the Spanish DGICYT (Project PB93-0157) for financial support of this work. Fellowship support to G. de la Puente by FICYT is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
M.V. Ko¨k, M.R. Pamir, J Anal Appl Pyrolysis 35 (1995) 145. J.P. Elder, M.B. Harris, Fuel 63 (1984) 262. J.H. Campbell, G. Gallegos, M. Gregg, Fuel 59 (1980) 727. P. Ungerer, R. Pelet, Nature 327 (6117) (1987) 52. R.J. Tyler, Fuel 59 (1980) 218. K.R. Doolan, J.C. Mackie, R.J. Tyler, Fuel 66 (1987) 572. M.A. Serio, W.A. Peters, J.B. Howard, Ind Eng Chem Res 26 (9) (1987) 1831. J.C. Crelling, R.H. Schrader, L.G. Benedict, Fuel 58 (1979) 542. A. Cagigas, J.B. Escudero, M.J.D. Low, J.J. Pis, J.M.D. Tasco´n, Fuel Proc Technol 15 (1987) 245. F.E. Huggins, G.P. Huffman, G.R. Dunmyre, M.J. Nardozzi, M.C. Lin, Fuel Proc Technol 15 (1987) 233. Ch. Song, A.K. Saini, H.H. Schobert, Energy Fuels 8 (1994) 301. S. Sarkar, Fuel 59 (1980) 450. J.W. Larsen, D. Lee, T. Scmidt, A. Grint, Fuel 65 (1986) 595. J.T. Senftle, A. Davis, Int J Coal Geol 3 (1984) 375. J.J. Pis, T.A. Centeno, M. Mahamud, A.B. Fuertes, J.B. Parra, J.A. Pajares, R. Bansal, Fuel Proc Technol 47 (1996) 119. ASTM. Method D720, Test for Free-Swelling Index of Coal; ASTM: Philadelphia, PA, 1991. T. Coll, J.F. Perales, J. Arnaldos, J. Casal, Thermochim Acta 196 (1992) 53. Brown ME, Introduction to Thermal Analysis: Techniques and Applications, Chapman and Hall, London, 1988, 134. D.B. Anthony, J.B. Howard, Am Inst Chem Eng J 22 (4) (1976) 625. D.S. Thakur, H.E. Nuttall, Ind Eng Chem Res 26 (1987) 1351. A. Mianowski, T. Radko, Fuel 72 (1993) 1537. M.K. Misra, R.H. Essenhigh, Energy Fuels 2 (1988) 371. Suuberg EM, Peters WA, Howard JB, The 17th Symposium (Int.) on Combustion. The Combustion Institute, Pittsburg, CA. 1979 117. J.J. Pis, G. de la Puente, E. Fuente, A. Mora´n, F. Rubiera, Thermochim Acta 279 (1996) 93. Habermehl D, Orywal F, Beyer HD, In: Elliot MA, editor. Chemistry of Coal Utilization, Wiley-Interscience, New York, 1981, 317. D.J. Maloney, R.G. Jenkins, P.L. Walker, Fuel 61 (1982) 175. J.J. Pis, A. Cagigas, P. Simon, J.J. Lorenzana, Fuel Proc Technol 20 (1988) 307. V.P. Ignashin, N.D. Rus’yanova, A.V. Tenina, Coke Chem (USSR) 1 (1990) 1. H.A.G. Chermin, D.W. van Krevelen, Fuel 36 (1957) 85. M.A. Serio, D.G. Hamblem, J.R. Markham, P.R. Solomon, Energy Fuels 1 (1987) 138. H. Ju¨ntgen, Fuel 63 (1984) 731. C.A. Rhoads, J.T. Senftle, M.M. Coleman, A. Davis, P.C. Painter, Fuel 62 (1983) 1387. de la Puente G, Fuente E, Iglesias MJ, Rubiera F, Pis JJ, The Ninth International Conference on Coal Science, Essen, Germany, 1997. Spiro CL, Internal Report General Electric, Schenectady, New York, 1981. A. Jurkiewicz, A. Marzec, N. Pislewski, Fuel 61 (1982) 647. A. Marzec, A. Jurkiewicz, N. Pislewski, Fuel 62 (1983) 996.
Modelling of volatile product evolution in coal pyrolysis. The role of aerial oxidation G. de la Puente, G. Marba´n, E. Fuente, J.J. Pis * Instituto Nacional del Carbo´n, CSIC. Apartado 73, 33080 O6iedo, Spain Received 14 September 1997; accepted 22 September 1997
Abstract Non-isothermal thermogravimetry has been applied to study the pyrolysis behaviour of coal and its changes as a consequence of aerial oxidation. A mathematical model has been developed to calculate the kinetic parameters of the coal pyrolysis process, which takes into account three different groups of thermal decomposition reactions for fresh coal, and two groups for oxidised coals. The agreement between the model predictions, for any heating rate, and the experimental values was fairly good. The volatile matter release profile of fresh coal can be described using three stages: Stage I (250 – 475°C), mainly light species are liberated; Stage II (475-575°C), characteristic of bituminous coals, high molecular weight species (tar) and hydrocarbons (primary gases) are evolved, which may lead to melting (metaplast); Stage III ( \575°C), secondary gases are produced while undergoing ring condensation and leading to the formation of coke. The decrease in the amount of aliphatic hydrocarbons released in the pyrolysis of oxidised coals implies that stage II is not observed, ‘mobile’ phase is insufficient to allow aromatic planar units to slide over each other, the melting cannot begin, and oxidised coals pass directly to stage III. This causes loss of plasticity and is therefore, responsible for the degradation of coking properties, as a consequence of oxidation. The aromatic fractions, that cannot rearrange, form a poor ordered structure and yield a powdered char. © 1998 Elsevier Science B.V. Keywords: Coal oxidation; Pyrolysis modelling; Reaction kinetics; Thermogravimetry; Differential thermal analysis
* Corresponding author. Tel.: +34 8 5280800; fax: + 34 8 5297662. 0165-2370/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 5 - 2 3 7 0 ( 9 7 ) 0 0 0 7 8 - 8
206
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
1. Introduction Coal devolatilisation is an intrinsic step in most coal conversion processes, whether it is gasification, combustion or carbonisation. In the course of the carbonisation process, as temperature increases important changes in structure and physical conditions of the solid phase occurs, resulting in the formation of the solid residue of carbonisation: coke or char. The application of the thermal analysis method to study the combustion and pyrolysis behaviour of fossil fuels has gained a wide acceptance among research workers, which is of exceptional significance for industry and the economy [1]. Elder and Harris [2] investigated the thermal characteristics of Kentucky bituminous coals undergoing pyrolysis in an inert atmosphere. The exothermic heat flow from 300 – 550°C, where the major weight loss occurs, has been associated with the primary carbonisation process and the development of the plastic state. The end of this stage is the onset of secondary gasification, which is responsible for coke formation. Mathematical models of hydrocarbon formation are based on first-order kinetics conforming to the Arrhenius equation. In such models [3,4], ‘n’ parallel and independent reactions were used to explain the formation of different products during the primary cracking in pyrolysis. With respect to secondary pyrolysis, Tyler [5] studied the devolatilisation of bituminous coals in a fluidized bed reactor in the temperature range between 500 and 900°C, Doolan et al. [6] studied the cracking reactions of tars generated in the flash pyrolysis of coal at 600 – 1700°C, and Serio et al. [7] studied the yield and kinetics of tars generated by several types of coal at 500–900°C. Coal oxidation occurs when coal is exposed to the air during the operations of extraction, preparation, transport and storage. Oxidation produces remarkable changes in coal properties [8 – 11], some of which have a negative effect in the industrial use of coal. For these reasons, the study of aerial oxidation of coals has received much attention in literature, the detrimental effect that oxidation has upon the coking and caking properties, through the loss of plastic properties, is well documented [12 – 15]. Changes in coal behaviour are mainly due to the chemical and structural modifications taking place during air oxidation. The objective of the present work is to develop a general model of coal pyrolysis that predicts the behaviour of coal and its changes during thermal treatment as a consequence of pre-oxidation. It is necessary to know the amount and evolution rate of gas species as a function of temperature. A knowledge of thermal reaction kinetics is necessary to understand and predict the pyrolysis behaviour of coals. A multistep pyrolysis model has been developed which takes into account three different groups of thermal decomposition reactions for fresh coal, whilst the pyrolysis of oxidised coals is described by means of just two reactions.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
207
2. Experimental A medium volatile bituminous coal, Amonate, was used in this work. The coal was dried under atmospheric pressure at 40°C, and then it was ground to a particle size below 150 mm. Representative coal samples were obtained using a small sample divider (riffle). Oxidation was carried out under air flow in a forced convection furnace at 200°C and these conditions were maintained for times varying from 6 h to 14 days in order to prepare samples with various oxidation degrees. The samples were uniformly distributed in trays in a thin layer, in order to have a similar air exposition for the particles. After oxidation, coal samples were stored under argon to avoid ulterior oxidation. Chemical characterisation (proximate and ultimate analyses) of fresh coal and oxidised samples was made following international standard procedures. The elemental analysis was carried out using LECO CHN 600 and LECO SC 132 equipment, and the oxygen content was calculated by the difference. Results are presented in Table 1. Differential thermal analysis was used to determine the pyrolysis behaviour of fresh and oxidised coals. Experiments were carried out in a Setaram TAG 24 thermal analysis system. Coal samples of 25 mg, nitrogen flow rate of 50 ml min − 1 and a linear heating rate of 50°C min − 1 were used. Additionally, some experiments were performed at different heating rates (25 and 75°C min − 1). The weight loss (thermogravimetric TG signal) and the rate of weight loss (differential thermogravimetric DTG signal) as a function of time or temperature were recorded while the coals where subjected to a controlled temperature program. Caking properties were determined using the free-swelling index standard test method ASTM D720-91 [16].
Table 1 Chemical characterisation of fresh coal and oxidised samples Oxidation time (days)
0 0.25 3 5 7 10 14 a b
Elemental compositiona (%)
Proximate analysis
Moisture (%)
Volatile mattera (%)
Ashb (%)
C
H
N
S
Odiff
0.5 1.8 1.2 1.8 2.6 2.7 1.2
25.1 23.2 27.0 29.7 32.0 32.4 33.4
6.0 8.2 6.9 8.0 6.6 6.7 6.7
90.3 82.2 79.6 78.0 76.5 76.2 75.6
4.9 2.7 2.6 2.2 2.2 2.2 2.0
1.4 1.5 1.3 1.4 1.2 1.3 1.2
0.8 0.7 0.7 0.7 0.7 0.7 0.6
2.6 12.9 15.8 17.7 19.4 19.7 20.6
Dry ash free basis (daf). Dry basis.
208
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
3. Results and discussion The rate of weight loss as a function of temperature for the parent coal is shown in Fig. 1. For fresh coal three temperature regions can be observed, which correspond to three groups of thermal decomposition reactions. After the loss of moisture, around 250°C the desorption of gases started. At about 525°C the rate of thermal decomposition reached a maximum. A subsequent final phase of degasification was observed. These results are in good agreement with those of other authors [17]. The first region corresponds to the loss of light molecules which are linked physically to the carbonaceous matrix. At higher temperatures the breaking of thermal decomposition bonds occurs, originating the formation of tar and hydrocarbons. Finally, at higher temperatures the pyrolysis is predominantly characterised by the formation of hydrogen. In the case of bituminous coals, they can melt during the plastic range (350– 500°C). Afterwards the conversion of semicoke to high-temperature coke, characterised by the formation of a more or less regular structure at temperatures above 700°C takes place. From the shape of the thermogravimetric curve DTG (Fig. 1) the presence of three stages in the pyrolysis of fresh coal can be deduced: Stage I, between 250 and 475°C, during which the weight loss is small (B 0.25% s − 1). Stage II, between 475 and 575°C, involving a very important weight loss for the fresh bituminous coal. Stage III, above 575°C, this implies a relatively small weight loss (B 0.25% s − 1).
Fig. 1. Rate of weight loss in the pyrolysis of fresh coal.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
209
Fig. 2. Rate of weight loss in the pyrolysis of the coals oxidised for various periods of time (from 0.25 to 14 days).
When the oxidised coals were considered (Fig. 2), two peaks were observed. The most important change in DTG curves as a consequence of coal oxidation was the drastic decrease in the rate of weight loss around 500°C. At this temperature, where fresh coal presents the maximum, the oxidised coals show an inflexion point. This result indicates that stage 2 of pyrolysis has practically disappeared for oxidised coals. From the shape of these thermogravimetric curves (DTG) the presence of only two stages in the pyrolysis of oxidised coals can be inferred: Stage I. The weight loss is more important than for fresh coal, and it increases as the oxidation degree increases. After 7 days of treatment almost no changes were observed. Stage III. The oxidised samples show a much more important volatile release at this stage, but an increase in the oxidation time causes a decrease in the relative quantity of volatile matter evolved at these temperatures.
3.1. Kinetics of thermal decomposition The kinetic parameters of the thermal decomposition reactions were calculated using a differential method assuming that: (i) various parallel reactions exist [18]; (ii) the kinetic equations of each reaction are of first order [19–21]; and the rate constants follow Arrhenius’ law. The model proposed in this work considers that the components of coal can evolve as non-competitive independent reactions [22]. The following expression can therefore be established:
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
210
dXi =ki (1 − Xi ) dt
(1)
where i indicates reactions 1, 2 or 3, and Xi is the fraction of volatile matter released during reaction i: Xi =
mti −mi mti
(2)
where mti is the total weight of volatiles that evolved during reaction i, and mi is the weight of the remaining volatiles that are released in reaction i at time t and temperature T. The kinetic constant ki can be expressed, according to Arrhenius’ law: ki = A i e
−
Ei RT
(3)
where Ei and Ai represent the activation energy and the pre-exponential factor for the reaction i. This model assumes that during each reaction i a fraction fi of the total volatile matter is released. The mathematical model is based on the calculus of the parameters fi, Ei and Ai that give the same weight as the experimental data obtained in the thermobalance. At a time t the fraction Xi was calculated from:
&
Xi
Xij
dXi = Ai (1 − Xi )
&
t
e
−
Ei RT
dt
(4)
tj
&
where Xij is the fraction Xi at a time tj lower than t. Xi =1 − exp ln(1 −Xij ) −
t
Ai e
−
Ei RT
dt
(5)
tj
With the obtained values of Xi the weight loss at a time t was calculated: n
m = mo − % Xi fi mt
(6)
i=1
where m0 is the initial weight of sample and mt the total weight evolved during the pyrolysis process, data obtained in the thermogravimetric analyser. The values of weight as a function of time, obtained from the model were compared with experimental data (TG curve). The rate of weight loss was calculated: −
n E dm − i = mt % fi (1 − Xi )Ai e RT dt i=1
(7)
and the results obtained from the multistep kinetic model were tested by comparing the experimental and simulated DTG curves. Taking into account the above discussed shape of thermogravimetric curves (Figs. 1 and 2), it was assumed that volatile matter was released in three parallel reactions for fresh coal, and two reactions for coals oxidised at various degrees.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
211
Fig. 3. Comparison between experimental data and theoretical values predicted using the kinetic model for fresh coal.
An illustration of the experimental rate of weight loss, − dm/dt, and the curves obtained from the application of the kinetic model for the global process and three independent reactions considered for fresh coal, is given in Fig. 3. The agreement between the predicted and experimental data is quite good. The kinetic parameters obtained are presented in Table 2, where Tmi represents the temperature of maximum weight loss during reaction i. It is observed that during reactions 2 and 3 most of the volatile compounds (95%) contained in fresh coal were liberated. Due to the great complexity of the reactions taking place during pyrolysis it is not possible, without further analysis, to determine from the obtained kinetic data which compounds are produced in each reaction. Based on the similarity of activation energy and Arrhenius factor values obtained for reaction 2 and those reported by Suuberg [23] for the formation of light aliphatic hydrocarbons during the pyrolysis of bituminous coal, the results obtained from this model indicate that this kind of compounds are produced at temperatures of 475–575°C. Table 2 Kinetic parameters obtained for fresh coal
Tmi (°C) Ei (kcal mol−1) Ai (s−1) fi (%)
Reaction 1
Reaction 2
Reaction 3
394 25.7 6.33×106 4.4
524 46.4 1.62×1011 50.7
650 10.5 1.64 44.9
212
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
Fig. 4. Comparison between experimental data and theoretical values predicted using the kinetic model for coal oxidised at 200°C for 7 days.
Fig. 4 gives a general comparison of volatile release profiles for oxidised coals, the experimental rate of weight loss, − dm/dt, and the curves obtained from the application of the kinetic model for the global process and the two independent reactions considered for the coal oxidised during 7 days at 200°C. In all cases a very good agreement was found between experimental and predicted data. The obtained kinetic parameters are given in Table 3. No significant differences were observed, indicating that pyrolysis takes place using a similar pathway for coal oxidised at various degrees. The amount of volatile matter released during reaction 1 increased as the oxidation degree increased. Both the kinetic parameters and the temperature of the maximum weight loss, Tm3, of reaction 3 for fresh coal are very similar to those of reaction 3 for the oxidised coals, the difference being the yield increased from 45% for fresh coal to 90% for the oxidised coals. Reaction 2 corresponds to fresh coal, with a maximum weight loss at 524°C, this does not appear for the pyrolysis of oxidised coals. The low energy produced in the combustion of volatile matter of oxidised coals [24] compared to that of fresh coals confirms that fresh coal liberates light hydrocarbons (reaction 2) with a higher heating value than volatile matter produced from oxidised coals, with a higher CO2 and H2O content. It has been reported that pre-oxidation at low temperatures (B 300°C) in air can reduce or even destroy the thermoplastic properties of coals [25–27]. Caking properties were determined using the free-swelling index standard test method. The fresh sample presented a free swelling index of 8, and swelling disappeared after 6 h of oxidation, giving a free swelling index of 0 for the rest of the series. According
Tm1 (°C) 372 431 424 434 418 429
tox (days) 0.25 1 5 7 10 14
15.2 16.8 19.1 16.0 16.4 18.5
E1 (kcal mol−1)
Table 3 Kinetic parameters obtained for oxidised coals
2.33 2.50 1.67 1.17 2.30 3.03
A1×10−3 (s−1) 2.1 3.9 8.4 8.3 8.7 8.8
f1 (%) 652 638 645 620 622 626
Tm3 (°C)
11.4 11.0 11.8 10.3 10.2 10.3
E3 (kcal mol−1)
2.73 2.50 3.95 1.81 1.72 1.82
A3×10−3 (s−1)
97.9 96.1 91.6 91.7 91.3 91.2
f3 (%)
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218 213
214
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
Table 4 Kinetic parameters calculated for coal pyrolysed at different heating rates (tox =10 days) l (°C min−1)
E1 (kcal mol−1)
A1 · 10−3 (s−1)
f1 (%)
E3 (kcal mol−1)
A3×10−3 (s−1)
f3 (%)
25 50 75
16.4 16.4 16.4
1.15 2.30 3.45
8.7 8.7 8.7
10.2 10.2 10.2
0.86 1.72 2.58
91.3 91.3 91.3
to this, the results are in agreement with the hypothesis [28] that the reduction of the main pyrolysis peak in the DTG curve is proportional to the degradation of plastic properties. Based on the absence of reaction 2, corresponding to the formation of light aliphatic hydrocarbons from bituminous coals, it seems reasonable to assume that the kind of volatile products in the oxidised coals, almost free of light aliphatic hydrocarbons, is the cause of the loss of plasticity and, is therefore responsible for the degradation of coking properties of coals as a consequence of oxidation. The proposed model gives details on the different behaviour of the coals during pyrolysis. The validation of this model was based on its ability to predict the volatile release profiles at different heating rates than that used for its proposition. Experiments were carried out for coal oxidised during 10 days at heating rates, l, of 25, 50 and 75°C min − 1. The kinetic parameters were calculated using the proposed model, and the results are given in Table 4. In Fig. 5 the rates of volatile
Fig. 5. Volatile release profiles at different heating rates, comparison between predicted values (lines) and experimental data (symbols).
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
215
release are plotted against temperature for each heating rate, and the predicted data are compared with the experimental ones. For all heating rates the same values were obtained both for the activation energies, Ei, and for the relative fractions evolved, fi, corroborating the applicability of the model. A linear dependency of the Arrhenius factor, Ai, with the heating rate, l, is observed and the temperatures of maximum rate of weight loss, Tmi, are independent of the heating rate.
3.2. Macroscopic pyrolysis model According to the obtained results, coal pyrolysis can be described in terms of the stages shown in Fig. 6. This view is an extension of a model originally proposed by Chermin and van Krevelen [29], modified by Serio [30] and adapted in this work. When a fresh bituminous coal is considered (continuous lines and gross characters in Fig. 6): During stage I some light species, that exist as guest molecules or are formed by breaking of very weak bonds, are released (light species). During stage II, characteristic of bituminous coals, further bond-breaking and reduction of hydrogen bonding occurs, leading to evolution of high molecular weight species (tar) and hydrocarbons (primary gases), and lead to melting (metaplast). During stage III, the products can continue to react; the metaplast can evolve secondary gases, mainly CO and H2, while undergoing ring condensation and leading to the formation of coke. In the case of oxidised bituminous coals (dotted lines and underlined characters in Fig. 6): In stage I, (Figs. 1 and 2) an increase in the proportion of volatile products released at temperatures around 400°C was observed. The weight loss at these temperatures is attributed to the diffusion of molecules with a low molecular weight (B150–200) through the porous network of the coal [31]. Oxidation creates functional groups in the structure of the coal that can be more easily broken than
Fig. 6. Different stages of the pyrolysis process for fresh (solid lines and gross characters) and oxidised bituminous coals (dotted lines and underlined characters).
216
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
those of fresh coal. This observation is in line with the example that C–H bond strength is 103 kcal mol − 1 compared with 83, 78 and 91 kcal mol − 1 for the C–O, R –O –R and R – OH bonds, respectively. The presence of these functional groups in oxidised coals facilitates greater decomposition at low temperatures, that will be released mainly as CO and CO2. Taking into account that aliphatic CH groups have been reported as the key factor in the development of fluidity [32], the big decrease of this functionality observed in the FT-IR spectrum of the Amonate coal oxidised at 200°C [33] can explain the destruction of the plastic properties of this coal. Stage II concludes when it has consumed all the available hydrogen from the aliphatic and hydroaromatic fraction of the coal to saturate the free radicals produced. The big decrease in the amount of aliphatic hydrocarbons produced means that reaction 2 is not observed, melting can not begin, and oxidised coals pass directly to stage III. At stage III, ring condensation begins above 575°C, with the elimination of hydrogen. At high temperatures, CO is produced from the cracking of oxygenated functions. The aromatic fractions, in the case of oxidised coals are not bound, these give rise to the formation of a powdered char. The development of fluidity in coal is still poorly understood. It has been suggested that both the number and the nature of cross-links in the coal are key factors in the process. To illustrate the effect of oxidation, a macroscopic view of coal plasticity is presented in Fig. 7, based on one given by Spiro [34] and modified in this work. On the left side the stages for fresh coal are presented, whilst the right side gives an outline of the stages oxidised coal undergoes. According to this
Fig. 7. Macroscopic view of coal plasticity.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
217
macroscopic view, a bituminous coal can be described as a network of macromolecules with trapped molecules (Fig. 7A). Many authors distinguish a ‘mobile’ and a ‘immobile’ phase in coal [35,36]. The mobile phase consists of trapped molecules and fragments of the network. Upon heating, the macromolecules decompose and the aromatic planar units can slide over each other, in a more or less free way (Fig. 7B). This is a quite liquid state, in which the coal exhibits viscoelastic properties. The molecules, which are initially trapped, are free now and can act as lubricants. Concurrently, thermal decomposition at progressively higher temperatures leads to evolution of gases and tars. These internally generated volatile components are trapped within the viscous mass and cause swelling. At a particular temperature the expanded mass solidifies (Fig. 7C), due to the escape of gaseous molecules, the lack of lubricants, and the occurrence of condensation reactions, gives rise to the formation of coke. The volatile matter released from oxidised coals, almost free of light aliphatic hydrocarbons, as discussed above on the disappearance of stage II of pyrolysis, means that not enough ‘mobile’ phase is formed to allow aromatic planar units to slide over each other due to a lack of lubricants, and is the cause of the destruction of the thermoplastic properties of coals. The aromatic fractions, that in the case of oxidised coals cannot rearrange, form a poorly ordered structure (Fig. 7D) and yield a powdered char.
4. Conclusions From the shape of the thermogravimetric curve (DTG) the presence of three stages in the pyrolysis of fresh bituminous coal can be deduced. The most important change in DTG curves as a consequence of coal oxidation was the drastic decrease in the rate of weight loss around 500°C. From the shape of these thermogravimetric curves (DTG) the presence of only two stages in the pyrolysis of oxidised bituminous coals can be inferred. The kinetic parameters of thermal decomposition reactions were calculated using a differential method considering that the components of coal can evolve due to non-competitive independent reactions. The model was tested using experimental data and the agreement found was fairly good. Stage II, corresponds to the formation of light aliphatic hydrocarbons from bituminous coals, this does not appear for the pyrolysis of oxidised coals. To illustrate the effect of oxidation, a macroscopic view of coal plasticity is presented. A bituminous coal can be described as a network of macromolecules with trapped molecules in it. Upon heating the macromolecules decompose and the aromatic planar units can slide over each other, more or less free. This is a quite liquid state, in which the coal exhibits viscoelastic properties. The decrease in the amount of aliphatic hydrocarbons released in the pyrolysis of oxidised coals means that stage II is not observed, not enough ‘mobile’ phase is formed to allow aromatic planar units to slide over each other, the melting cannot begin, and oxidised coals pass directly to stage III. This causes the loss of plasticity and, therefore is responsible for the degradation of coking properties of coals as a consequence of oxidation.
G. de la Puente et al. / J. Anal. Appl. Pyrolysis 44 (1998) 205–218
218
Acknowledgements The authors wish to thank the Spanish DGICYT (Project PB93-0157) for financial support of this work. Fellowship support to G. de la Puente by FICYT is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
M.V. Ko¨k, M.R. Pamir, J Anal Appl Pyrolysis 35 (1995) 145. J.P. Elder, M.B. Harris, Fuel 63 (1984) 262. J.H. Campbell, G. Gallegos, M. Gregg, Fuel 59 (1980) 727. P. Ungerer, R. Pelet, Nature 327 (6117) (1987) 52. R.J. Tyler, Fuel 59 (1980) 218. K.R. Doolan, J.C. Mackie, R.J. Tyler, Fuel 66 (1987) 572. M.A. Serio, W.A. Peters, J.B. Howard, Ind Eng Chem Res 26 (9) (1987) 1831. J.C. Crelling, R.H. Schrader, L.G. Benedict, Fuel 58 (1979) 542. A. Cagigas, J.B. Escudero, M.J.D. Low, J.J. Pis, J.M.D. Tasco´n, Fuel Proc Technol 15 (1987) 245. F.E. Huggins, G.P. Huffman, G.R. Dunmyre, M.J. Nardozzi, M.C. Lin, Fuel Proc Technol 15 (1987) 233. Ch. Song, A.K. Saini, H.H. Schobert, Energy Fuels 8 (1994) 301. S. Sarkar, Fuel 59 (1980) 450. J.W. Larsen, D. Lee, T. Scmidt, A. Grint, Fuel 65 (1986) 595. J.T. Senftle, A. Davis, Int J Coal Geol 3 (1984) 375. J.J. Pis, T.A. Centeno, M. Mahamud, A.B. Fuertes, J.B. Parra, J.A. Pajares, R. Bansal, Fuel Proc Technol 47 (1996) 119. ASTM. Method D720, Test for Free-Swelling Index of Coal; ASTM: Philadelphia, PA, 1991. T. Coll, J.F. Perales, J. Arnaldos, J. Casal, Thermochim Acta 196 (1992) 53. Brown ME, Introduction to Thermal Analysis: Techniques and Applications, Chapman and Hall, London, 1988, 134. D.B. Anthony, J.B. Howard, Am Inst Chem Eng J 22 (4) (1976) 625. D.S. Thakur, H.E. Nuttall, Ind Eng Chem Res 26 (1987) 1351. A. Mianowski, T. Radko, Fuel 72 (1993) 1537. M.K. Misra, R.H. Essenhigh, Energy Fuels 2 (1988) 371. Suuberg EM, Peters WA, Howard JB, The 17th Symposium (Int.) on Combustion. The Combustion Institute, Pittsburg, CA. 1979 117. J.J. Pis, G. de la Puente, E. Fuente, A. Mora´n, F. Rubiera, Thermochim Acta 279 (1996) 93. Habermehl D, Orywal F, Beyer HD, In: Elliot MA, editor. Chemistry of Coal Utilization, Wiley-Interscience, New York, 1981, 317. D.J. Maloney, R.G. Jenkins, P.L. Walker, Fuel 61 (1982) 175. J.J. Pis, A. Cagigas, P. Simon, J.J. Lorenzana, Fuel Proc Technol 20 (1988) 307. V.P. Ignashin, N.D. Rus’yanova, A.V. Tenina, Coke Chem (USSR) 1 (1990) 1. H.A.G. Chermin, D.W. van Krevelen, Fuel 36 (1957) 85. M.A. Serio, D.G. Hamblem, J.R. Markham, P.R. Solomon, Energy Fuels 1 (1987) 138. H. Ju¨ntgen, Fuel 63 (1984) 731. C.A. Rhoads, J.T. Senftle, M.M. Coleman, A. Davis, P.C. Painter, Fuel 62 (1983) 1387. de la Puente G, Fuente E, Iglesias MJ, Rubiera F, Pis JJ, The Ninth International Conference on Coal Science, Essen, Germany, 1997. Spiro CL, Internal Report General Electric, Schenectady, New York, 1981. A. Jurkiewicz, A. Marzec, N. Pislewski, Fuel 61 (1982) 647. A. Marzec, A. Jurkiewicz, N. Pislewski, Fuel 62 (1983) 996.