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Volatiles release and porosity evolution during high pressure coal pyrolysis
Fuel 82 (2003) 285–292 www.fuelfirst.com
Volatiles release and porosity evolution during high pressure coal pyrolysisq Jerzy Tomeczek*, Stanisław Gil Department of Process Energy, Silesian Technical University, ul Krasin´skiego 8, 40-019 Katowice, Poland Received 15 January 2002; revised 20 June 2002; accepted 18 July 2002; available online 9 September 2002
Abstract The kinetics of porosity and of porous structure evolution have been examined during enhanced pressure (0.1 –3.0 MPa) pyrolysis for two subbituminous coals on the basis of volatiles release. It has been found that both the volatiles mass release and char porosity decrease with pyrolysis pressure. The behaviour of three pore size groups, 5 – 30, 30 – 300 and 300– 5000 nm, were observed during pyrolysis. An almost total elimination of the first group (5 – 30 nm) was found so that the porosity build up was entirely within the third size group (300– 5000 nm), particularly at 1 MPa pressure. During the stage of most intensive devolatilization the release of volatiles precedes the porosity evolution. q 2002 Published by Elsevier Science Ltd. Keywords: Pressurized pyrolysis; Kinetics of devolatilization; Porous structure
1. Introduction The efficiency of fuel conversion in power industry can be enhanced as a result of pressurized combustion when applied to combined gas –steam cycles, thus leading to carbon dioxide emission abatement. The long-term future of coal for power generation depends then on the development of pressurized coal combustion technologies in which the coal pyrolysis is a very important phase determining the pollutants formation. Mathematical modelling of coal nitrogen conversion during combustion requires data concerning the rate of volatiles release and the evolution of coal porous structure. However, investigations of volatiles release during rapid coal heating have a long history, the experimental evidence is contradictory. Badzioch and Hawksley [1] reported considerable enhancement of the amount of volatiles produced during coal pyrolysis in a drop tube. Knill et al. [2,3] found that in a drop tube up to 80% more volatiles are released than in standard pyrolysis conditions, nevertheless large divergence of results does not allow general conclusion to be drawn. Bonn et al. [4] also confirmed a strong enhancement of volatiles release. On the other hand, * Corresponding author. Tel./fax: þ 48-32-255-56-16. E-mail address: [email protected] (J. Tomeczek). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 5 6 - 9
Heyd [5] did not find clear evidence that increasing the rate of heating from 102 to 104 K s21 leads to volatiles enhancement, while the modelling by Niksa [6] predicts for the same coal an increase in volatiles, but only by 7%. The influence of the heating rate on the ultimate volatiles yields is then controversial and most probably the experimental method influences the results. In drop tubes used by Badzioch and Hawksley [1], Knill et al. [2,3], Bonn et al. [4] and others it is very difficult to close the sample mass balance that gives on one hand a strong enhancement of the volatiles yield and on the other the experiments have poor reproducibility. Methods in which the mass of the solid sample is carefully controlled demonstrate only small or even no influence of the heating rate on the amount of volatiles [7,8]. Less controversial is the influence of pressure on the pyrolysis. In all the reported experiments the volatiles release decreases with pressure [8 –12]. During rapid coal pyrolysis the residence time of volatiles and the pressure within the coal particle influence the secondary reactions involving hydrocarbons, thus the transport of volatiles from the micropores to the particle surface must be considered among the controlling phenomena. The success of mathematical modelling depends then on proper understanding of the coal porosity evolution, however, because of the experimental difficulties, only few experiments on kinetics of porosity are available. Simons [13] proposed that the rate of porosity evolution proceeds in
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ation products is already present in the original coal, and the accessibility of the micropore system increases owing the formation of new transport pores. They found during pyrolysis of a coking coal an increase by 150% of the micropore volume, which for a noncoking coal was halved, while for the lignite only a slight increase in microporosity was observed. There are no published data presenting the influence of pyrolysis pressure on the char porosity. The aim of the paper is to present the results of investigation of the pressure influence on the kinetics of coal devolatilization and char porous structure evolution. Applying the same technique of heating and sample removal from the reactor enabled reliable comparison of the kinetics of volatiles release and porosity build up. The porosity variation of three pore size groups at 0.1 and 1 MPa pyrolysis pressure was investigated. The influence of the pyrolysis pressure on the mass of volatiles release and the final total porosity of the char particles was studied at pressures up to 3 MPa.
Nomenclature A C dp E H k0 m N R S T t W 1 D1
ash content in coal carbon content in coal diameter of pores (m) activation energy (kJ mol21) hydrogen content in coal preexponential factor (s21) mass of solid sample (kg) nitrogen content in coal gas constant (kJ mol21 K21) sulphur content in coal temperature (K) time (s) water content in coal solid particle porosity solid particle porosity variation
Subscripts j 1
index of chemical reaction denotes the value for time t ! 1
2. Experiments
Superscripts a waf wf
analytical state water ash free state water free state
parallel with the volatiles release, that, however, has never been experimentally substantiated. Most research has concentrated on the structure of the final char after slow or fast heating. It has been found, that both the parent coal and the conditions of pyrolysis determine the final char structure, but no general relations could be found. Gavalas and Wilks [14] found for a nonsoftening coal a considerable decrease and for softening coals even a total elimination of the micropore volume. Singla et al. [15] observed an increase in micropores with the pyrolysis temperature. Matsuo et al. [16] found for low temperature pyrolysis of Yallourn brown coal that the micropore size distribution was not significantly changed, while for high temperature pyrolysis a remarkable increase of the microporosity was observed due to the release of inorganic gases after the tar formation stage. Weishauptova et al. [17] stated that the predominant part of the micropores detected in carboniz-
Three samples of two subbituminous coals SIERSZA and NIWKA were used during the investigations. The characteristics of the coals are presented in Table 1. The enriched samples were obtained by hand separation of the mineral matter from a large coal lump prior to grinding. A single layer of a narrow size range 0.9– 1.0 mm coal sample of 0.5 g mass was devolatilized in a reactor (Fig. 1) at heating rates up to 100 K s21, final heating temperature up to 1373 K and pyrolysis pressure up to 3 MPa. To avoid large temperature differences within the coal particles, the coal sample was heated from both the bottom and the top. The rate of heating to the final temperature and the residence time of the sample in this temperature were automatically controlled. A stream of 50 1 h21 helium was stabilized during the experiments. Within the whole experimental time of 90 s at various stages of pyrolysis the heating power was cut-off and the sample cooled by the flowing helium. This made possible the removal of the solid samples from the reactor for analysis. Data collected during the stage of linear heating at 5 or 2 s intervals and during isothermal holding at the final temperature at 15 s intervals, enabled the separation of the time and the temperature influences on the evaluated kinetic equations. The mass loss, composition and the porous structure of the samples removed from the reactor were then
Table 1 Characteristics of investigated coals Coal
W a (%)
V waf (%)
A wf (%)
C waf (%)
H waf (%)
N waf (%)
S a (%)
Real density (g cm23)
Initial porosity (vol%)
SIERSZA raw SIERSZA enriched NIWKA enriched
7.3 2.6 1.9
32.8 37.7 33.8
19.8 6.9 6.0
69.7 77.0 79.5
5.8 5.1 4.1
1.2 1.5 1.6
2.1 2.5 1.7
1.42 1.29 1.36
13.34 15.82 16.40
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Fig. 2. Coal sample mass loss as a function of pressure (enriched coal SIERSZA).
Fig. 1. Experimental unit.
measured. The structural investigations were carried out by means of a mercury porosimeter in which a pressure of 0.01 –200 MPa could be applied, allowing analyses of the pore size from 5 to 5000 nm. The correction for the modification of the coal and char structure by the mercury pressure was evaluated with the following parameters: coal compressibility, 1.6 £ 10210 m2 N21 [18]; mercury compressibility, 0.41 £ 10210 m2 N21 [18]; contact angle mercury/coal, 1358 [18]; mercury surface tension, 0.481 N m21 [19]. At each pyrolysis condition, three experiments were conducted and the final results of porosity and volatiles release were taken as an arithmetic mean value of three measurements. The error of the experimental method has been estimated as equal: for porosity 22.6% and for volatiles release 20.8%.
3. Results and discussion The influence of pressure on volatile mass release for enriched coal SIERSZA is presented in Fig. 2 for two rates of heating and two final heating temperatures. Within the analyzed temperature range for 0.1 MPa the amount of volatiles released is higher than that obtained under standard conditions (Table 1), the enhancement, however, does not exceed 30%. The previously described controversy concerning the amount of volatiles released during coal pyrolysis can be addressed for the lowest experimental pressure 0.1 MPa. The volatiles yield increase by 29% for the highest pyrolysis temperature 1373 K is caused partly by
fact that the V waf value in Table 1 was obtained for carbonization temperature 1123 K. For the lowest experimental temperature 1073 K, enhancement by 18% of volatiles mass was also found. However, the rate of heating applied during the experiments was not rapid, the yield of volatiles released from coal was higher than in standard carbonization conditions, even for the lowest temperature. The volatiles enhancement by about 30% at the highest temperature 1373 K is in close agreement with the results obtained by Loison and Chauvin [20] for a heating rate of 1500 K s21 to a final temperature of 1323 K. Because the last pyrolysis experiments [20] conducted on a metal gauze can certainly be qualified as rapid heating, and because the final pyrolysis temperatures differ only by 50 K, it may surely be concluded that fast pyrolysis does not enhance the release of volatiles by more than 30%. Also Anthony et al. [9], Suuberg et al. [21] and Cai et al. [8] reported volatiles yields increase only up to 25% during rapid pyrolysis (1000 K s21 to 1223 –1273 K) of small single layer coal samples in fixed bed reactors. What is more, Anthony et al. [9] found that within the heating rate 102 –104 K s21 the weight loss was independent of the heating rate. Determination of the devolatilization level from the change in ash level of the char, applied in most drop tube experiments during which volatiles enhancement by 80% was found [1 – 4], may be influenced by the gravitational separation of different density particles during the solids sampling process from the reactor. The agreement between the present fast and former rapid heating [8,9,20,28] experiments should not be regarded as incidental. In all cases the amount of volatiles was found on the basis of direct measurement of the sample mass loss, which is simple and accurate. It is very difficult to compare the influence of pressure on volatiles yield with the published data, mainly because of the different types of coal used but also the definition of the volatile mass release used by various authors (waf or analytical state) complicates the problem. Thus any comparison, such as presented in Fig. 3, can have only
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Fig. 3. Volatiles release as a function of pressure.
qualitative meaning. The observed strong influence of pressure on the volatiles release within the range 0.1– 1.5 MPa confirms the experiments of Anthony et al. [9] and Suuberg et al. [21] as well as the model of Niksa [12]. The escape of volatiles from a particle is a complex process involving volatiles formation and cracking to coke and light hydrocarbons within the particle and transport through a time-dependent pore structure. It has been previously observed [9,21] that the description of tar behaviour is the clue to the problem since tar is the main part of the volatiles. A strong drop in volatiles yield at moderate reactor pressure 0.1 –1.5 MPa most probably is an indication that pressure within the particle, determining the volatiles chemical reaction rate and transport, exceeds the reactor pressure by a comparable value. Tomeczek and Kowol [22] assuming a convective transport of volatiles through the porous structure found the pressure difference centre/surface within 1 mm particle size to be equal to about 0.1 MPa, confirming the last conclusion. This is the reason why in vacuum devolatilization the extent of the secondary reactions can be reduced due to enhanced volatiles transport, but only slightly above the 0.1 MPa reactor pressure conditions [9,21]. The total porosity variation of the carbonaceous particles for the same coal and under the same thermal conditions is presented in Fig. 4. Enhanced pressure decreases both the volatiles release as well the porosity variation. In all the examined cases an identical total pyrolysis time of 90 s was
Fig. 4. Porosity variation as a function of pressure (enriched coal SIERSZA).
used. The duration of the sample residence time at the final temperature is of particular importance when the experiments carried out under various conditions are compared, as shown in Table 2. During slow heating the final particle porosity is determined by the isothermal residence time at the final temperature. For fast heating, however, most of the porosity changes take place during the stage of intensive devolatilization but even in this case the isothermal period is important. Thus, in presenting porosity experiments it is necessary to inform not only about the rate of heating and the final temperature, but also about the exposure time at the final temperature. The comparison of the kinetics of volatiles release for the 0.1 and 1 MPa pyrolysis pressure is presented in Figs. 5 and 6. In both the cases the value of the maximum rate of volatiles release is similar, also the time position of this maximum can be considered independent of pressure. The rate of volatiles release has been described by the kinetic equation X Ej dmwaf waf ¼ k0j exp 2 Þ; ðmwaf 1j 2 m dt RT j¼1
ð1Þ
where the rate constant k0j ; Ej and mwaf 1j are given in Table 3 for the enriched coal SIERSZA. To describe well the experimental results it was necessary to assume up to six reactions at atmospheric pressure and five at 1.0 MPa. The curve calculated on the basis of the model for the SIERSZA
Table 2 Porosity as function of pyrolysis conditions (raw coal SIERSZA, 0.1 MPa) Final temperature (K)
Rate of heating
Porosity at time of reaching the final temperature (vol%)
Isothermal residence time in final temperature
Porosity after isothermal exposure (vol%)
673 873 1073 1373 1373 1373 1373
5 K min21 5 K min21 5 K min21 5 K min21 5 K min21 100 K s21 350 K s21
14.23 15.88 20.96 22.49 21.88 36.44 –
30 min 30 min 30 min 30 min 60 min 79 s 12 s
32.46 43.37 57.89 58.37 62.47 45.74 56.11
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Table 3 Kinetic constants in Eq. (1) for enriched coal SIERSZA Reaction j
1 2 3 4 5 6
Fig. 5. Measured and modelled rate of volatiles release (enriched coal SIERSZA).
in which the rate constants k0j ; Ej and 11j were evaluated on basis of the porosity measurements and are given in Table 4. For both analyzed pyrolysis pressures, five reactions had to
Fig. 6. Measured and modelled (kinetic constants Table 3) rate of volatiles release (enriched coal SIERSZA).
Ej (kJ mol21)
waf mwaf (%) 1j =m0
0.1 MPa
1 MPa
0.1 MPa
1 MPa
0.1 MPa
1 MPa
1 £ 105 1 £ 107 1 £ 107 1 £ 107 1 £ 108 1 £ 109
1 £ 105 1 £ 106 1 £ 107 1 £ 109 1 £ 1010 –
133 128 142 225 122 98
97 173 267 138 225 –
8.8 5.1 6.3 12.5 7.5 23.8
8.1 10.3 27.3 5.4 10.2 –
Table 4 Kinetic constants in Eq. (2) for raw (0.1 MPa) and enriched (1.0 MPa) coal SIERSZA Reaction j
coal given by Tomeczek and Kowol [22] for 0.1 MPa is presented in Fig. 5. The agreement between the present results and the modelling [20] is good, which certainly is due to similar coals being used in both the cases. Fig. 7 presents the comparison of the curve modelled by Eq. (1) with experimental points for the enriched coals SIERSZA and NIWKA. The agreement of the kinetic model with the experimental points is better for the SIERSZA than for NIWKA coal because the kinetic data in Table 3 were evaluated only on the basis of the first. The total porosity evolution has been modelled by a kinetic equation in which the values of the kinetic constants were assumed to be equal to that for volatiles release [13]. In the present study the porosity variation of the solid particles is described by X Ej d1 ¼ k0j exp 2 ð2Þ ð11j 2 1Þ; dt RT j¼1
k0j (s21)
1 2 3 4 5
k0j (s21)
Ej (kJ mol21)
11j (%)
0.1 MPa
1 MPa
0.1 MPa
1 MPa
0.1 MPa
1 MPa
1 £ 105 1 £ 107 1 £ 107 1 £ 108 1 £ 109
1 £ 105 1 £ 106 1 £ 107 1 £ 109 1 £ 109
131 127 145 105 306
132 201 257 169 182
9.2 2.4 7.8 3.5 41.2
11.1 13.1 32.8 3.5 2.9
be assumed in order to describe the experimental results. It can be seen that the kinetic constants given in Tables 3 and 4 are different, particularly for the last two reactions. Thus the assumption made by Simons [13] cannot be justified. Fig. 8 presents the modelling on the background of experiments for 1 MPa pyrolysis pressure. For both the coals the results are satisfactory. Comparison of Figs. 7 and 8 shows that the total particle porosity lags slightly behind the volatiles release, however, the values at maximum rate in both the cases are similar. Within the range of most intensive pyrolysis the delay times of porosity evolution for 1 MPa (Fig. 9) and for 0.1 MPa (Fig. 10) are similar. Most interesting, however, is the particle porous
Fig. 7. Measured and modelled rate of volatiles release.
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Fig. 11. Measured pore structure distribution (raw coal SIERSZA). Fig. 8. Measured and modelled rate of porosity variation.
groups and of the average pore size is presented in Fig. 13. The first size group undergoes drastic changes at short time and then remains relatively stable. The second pore size group increases gradually with time for 0.1 MPa pressure, while for 1 MPa this size pores remains almost stable. The largest evolution takes place in the pore size group 3, which for 0.1 MPa pressure developed gradually with time while for 1 MPa this pore size group after rapid increase at the beginning of pyrolysis continues to grow monotonically. The average pore size porosity increases almost linearly with time independent of pressure. In cases of heat and mass transfer modelling, knowledge of the pore size evolution is necessary. Simons [13] proposed a relation for the mean pore size time function Fig. 9. Measured and modelled rate of porosity variation and volatiles release (enriched coal SIERSZA).
structure. Figs. 11 and 12 present the porosity variation at 0.1 MPa and 1 MPa pyrolysis pressure for three pore size groups: 5 –30, 30– 300 and 300 –5000 nm. For both the values of the experimental pressure the total porosity evolution is the result of increase in only one size group 300 –5000 nm. The first group 5 – 30 nm almost totally collapsed. The time function of porosity of the three size
Fig. 10. Measured and modelled rate of porosity variation and volatiles release.
dp ðtÞ=dp ð0Þ ¼ ½1ðtÞ=1ð0Þ1=3 ;
ð3Þ
which, however, was not verified experimentally. Fig. 14 presents both the sides of this relation and additionally the function 1ðtÞ=1ð0Þ. It can be observed that the average pore diameter time function at atmospheric pressure is related quite well to the cubic root of porosity, while at 1 MPa pressure the average pore diameter increase is much stronger than the char porosity. Thus for 0.1 MPa pyrolysis pressure, the relation proposed by Simons must be taken
Fig. 12. Measured pore structure distribution (enriched coal SIERSZA).
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291
Fig. 13. Relationship for three pore size groups ð11;2;3 ðtÞ=11;2;3 ð0ÞÞ and average pore size ð1ðtÞ=1ð0ÞÞ for 20 K s21, 1073 K. A 11;2;3 ðtÞ=11;2;3 ð0Þ or 1ðtÞ=1ð0Þ; 0.1 MPa, raw coal SIERSZA, B 11;2;3 ðtÞ=11;2;3 ð0Þ or 1ðtÞ=1ð0Þ; 1.0 MPa enriched coal SIERSZA.
with caution. The cubic root relation at 0.1 MPa suggests a large contribution of spherical cavity pores within the char which were previously observed [23] for the parent coal.
4. Conclusions The kinetics of volatiles release and porosity evolution can be described by five reactions for which the activation energy depends on pressure. During the stage of most intensive devolatilization, the porosity evolution lags
behind the volatiles release. The delay time of porosity evolution was found to be independent of pyrolysis pressure. For the subbituminous coals analyzed, the total porosity evolution takes place within the macropores while the micropores almost collapse. At atmospheric pressure the average pore diameter time function related well to the cubic root of porosity, while at 1 MPa pressure the average pore diameter increase was much stronger. The average pore size porosity increased almost linearly with time for both 0.1 and 1 MPa pressure.
Fig. 14. Relationship between ½1ðtÞ=1ð0Þ1=3 ; 1ðtÞ=1ð0Þ and dp ðtÞ=dp ð0Þ for average pore size (20 K s21, 1073 K, raw coal SIERSZA 0.1 MPa and enriched coal SIERSZA 1 MPa). B dp ðtÞ=dp ð0Þ; p ½1ðtÞ=1ð0Þ1=3 ; A 1ðtÞ=1ð0Þ:
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The yield of volatiles during fast pyrolysis can be larger than that determined under standard conditions, but only by 30%. The final heating temperature and the residence time are the volatile yield controlling parameters during fast coal pyrolysis. Enhanced pressure decreased both the volatiles release and the total char porosity during coal pyrolysis. A strong drop of volatiles yield and porosity changes as pyrolysis pressure increased from 0.1 to 1.5 MPa was observed. The duration of the sample residence time at the final temperature is particularly important for particle porosity development during slow heating. For fast heating most of the porosity changes take place within the stage of intensive devolatilization but even in this case the isothermal period is important.
References [1] Badzioch S, Hawskley P. Ind Engng Chem Process Des Dev 1970;9: 521. [2] Knill K, Maalman T, Morgan M. Development of combustion characterization technique for high volatile bituminous coals. IFRF F88/a/10; 1989. [3] Knill K, Maalman T, Morgan M, Nel W. Characterization of the combustion behaviour of bituminous coals. IFRF F88/y/12; 1990. [4] Bonn B, Bauman H, Pauly W, Seewald H. Proceedings of the Eighth International Coal Science Conference, Oviedo. Amsterdam: Elsevier; 1995. p. 759.
[5] Heyd LE. MS Thesis. Department of Chemical Engineering, Princeton University; 1982. [6] Niksa S. 22nd Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1988. p. 105. [7] Niksa S, Heyd LE, Russel WB, Saville DA. 20th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1984. p. 1445. [8] Cai HY, Gu¨el AJ, Dugwell DR, Kandiyoti R. Fuel 1993;72:321. [9] Anthony DB, Howard JB, Hottel HC, Meissner HP. Fuel 1976;55:121. [10] Suuberg EM. ScD Thesis. MIT, Cambridge, MA; 1977. [11] Megaritis A, Zhuo YQ, Messenbo¨ck R, Dugwell DR, Kandiyoti R. Proceedings of the Ninth International Conference on Coal Science, Essen; 1997. p. 613. [12] Niksa S. Combust Flame 1995;100:384. [13] Simons GA. Combust Flame 1983;50:275. [14] Gavalas GR, Wilks KA. AICHE J 1980;26(2):201. [15] Singla PK, Miura S, Hudgima RR, Silveston PL. Fuel 1983;62:645. [16] Matsuo Y, Hayashi J-i, Kusakabe K, Morooka S. Proceedings of the Eighth International Conference on Coal Science, Oviedo. Amsterdam: Elsevier; 1995. [17] Weishauptova Z, Hedek J, Balek V, Sykorova I. Proceedings of the Eighth International Conference on Coal Science, Oviedo. Amsterdam: Elsevier; 1995. p. 933. [18] Spitzer Z. Powder Technol 1981;29:177. [19] Italiano P. Porosity—application analytical method. Milan: Carlo Erba Strumentazione Instruction; 1981. [20] Loison R, Chauvin R. Chim Ind 1964;91:269. [21] Suuberg EM, Peters WA, Howard JB. 17th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1978. p. 117. [22] Tomeczek J, Kowol J. Can J Chem Engng 1991;69:286. [23] Tomeczek J, Mlonka J. Fuel 1998;77:1841.
Volatiles release and porosity evolution during high pressure coal pyrolysisq Jerzy Tomeczek*, Stanisław Gil Department of Process Energy, Silesian Technical University, ul Krasin´skiego 8, 40-019 Katowice, Poland Received 15 January 2002; revised 20 June 2002; accepted 18 July 2002; available online 9 September 2002
Abstract The kinetics of porosity and of porous structure evolution have been examined during enhanced pressure (0.1 –3.0 MPa) pyrolysis for two subbituminous coals on the basis of volatiles release. It has been found that both the volatiles mass release and char porosity decrease with pyrolysis pressure. The behaviour of three pore size groups, 5 – 30, 30 – 300 and 300– 5000 nm, were observed during pyrolysis. An almost total elimination of the first group (5 – 30 nm) was found so that the porosity build up was entirely within the third size group (300– 5000 nm), particularly at 1 MPa pressure. During the stage of most intensive devolatilization the release of volatiles precedes the porosity evolution. q 2002 Published by Elsevier Science Ltd. Keywords: Pressurized pyrolysis; Kinetics of devolatilization; Porous structure
1. Introduction The efficiency of fuel conversion in power industry can be enhanced as a result of pressurized combustion when applied to combined gas –steam cycles, thus leading to carbon dioxide emission abatement. The long-term future of coal for power generation depends then on the development of pressurized coal combustion technologies in which the coal pyrolysis is a very important phase determining the pollutants formation. Mathematical modelling of coal nitrogen conversion during combustion requires data concerning the rate of volatiles release and the evolution of coal porous structure. However, investigations of volatiles release during rapid coal heating have a long history, the experimental evidence is contradictory. Badzioch and Hawksley [1] reported considerable enhancement of the amount of volatiles produced during coal pyrolysis in a drop tube. Knill et al. [2,3] found that in a drop tube up to 80% more volatiles are released than in standard pyrolysis conditions, nevertheless large divergence of results does not allow general conclusion to be drawn. Bonn et al. [4] also confirmed a strong enhancement of volatiles release. On the other hand, * Corresponding author. Tel./fax: þ 48-32-255-56-16. E-mail address: [email protected] (J. Tomeczek). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 2 5 6 - 9
Heyd [5] did not find clear evidence that increasing the rate of heating from 102 to 104 K s21 leads to volatiles enhancement, while the modelling by Niksa [6] predicts for the same coal an increase in volatiles, but only by 7%. The influence of the heating rate on the ultimate volatiles yields is then controversial and most probably the experimental method influences the results. In drop tubes used by Badzioch and Hawksley [1], Knill et al. [2,3], Bonn et al. [4] and others it is very difficult to close the sample mass balance that gives on one hand a strong enhancement of the volatiles yield and on the other the experiments have poor reproducibility. Methods in which the mass of the solid sample is carefully controlled demonstrate only small or even no influence of the heating rate on the amount of volatiles [7,8]. Less controversial is the influence of pressure on the pyrolysis. In all the reported experiments the volatiles release decreases with pressure [8 –12]. During rapid coal pyrolysis the residence time of volatiles and the pressure within the coal particle influence the secondary reactions involving hydrocarbons, thus the transport of volatiles from the micropores to the particle surface must be considered among the controlling phenomena. The success of mathematical modelling depends then on proper understanding of the coal porosity evolution, however, because of the experimental difficulties, only few experiments on kinetics of porosity are available. Simons [13] proposed that the rate of porosity evolution proceeds in
286
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ation products is already present in the original coal, and the accessibility of the micropore system increases owing the formation of new transport pores. They found during pyrolysis of a coking coal an increase by 150% of the micropore volume, which for a noncoking coal was halved, while for the lignite only a slight increase in microporosity was observed. There are no published data presenting the influence of pyrolysis pressure on the char porosity. The aim of the paper is to present the results of investigation of the pressure influence on the kinetics of coal devolatilization and char porous structure evolution. Applying the same technique of heating and sample removal from the reactor enabled reliable comparison of the kinetics of volatiles release and porosity build up. The porosity variation of three pore size groups at 0.1 and 1 MPa pyrolysis pressure was investigated. The influence of the pyrolysis pressure on the mass of volatiles release and the final total porosity of the char particles was studied at pressures up to 3 MPa.
Nomenclature A C dp E H k0 m N R S T t W 1 D1
ash content in coal carbon content in coal diameter of pores (m) activation energy (kJ mol21) hydrogen content in coal preexponential factor (s21) mass of solid sample (kg) nitrogen content in coal gas constant (kJ mol21 K21) sulphur content in coal temperature (K) time (s) water content in coal solid particle porosity solid particle porosity variation
Subscripts j 1
index of chemical reaction denotes the value for time t ! 1
2. Experiments
Superscripts a waf wf
analytical state water ash free state water free state
parallel with the volatiles release, that, however, has never been experimentally substantiated. Most research has concentrated on the structure of the final char after slow or fast heating. It has been found, that both the parent coal and the conditions of pyrolysis determine the final char structure, but no general relations could be found. Gavalas and Wilks [14] found for a nonsoftening coal a considerable decrease and for softening coals even a total elimination of the micropore volume. Singla et al. [15] observed an increase in micropores with the pyrolysis temperature. Matsuo et al. [16] found for low temperature pyrolysis of Yallourn brown coal that the micropore size distribution was not significantly changed, while for high temperature pyrolysis a remarkable increase of the microporosity was observed due to the release of inorganic gases after the tar formation stage. Weishauptova et al. [17] stated that the predominant part of the micropores detected in carboniz-
Three samples of two subbituminous coals SIERSZA and NIWKA were used during the investigations. The characteristics of the coals are presented in Table 1. The enriched samples were obtained by hand separation of the mineral matter from a large coal lump prior to grinding. A single layer of a narrow size range 0.9– 1.0 mm coal sample of 0.5 g mass was devolatilized in a reactor (Fig. 1) at heating rates up to 100 K s21, final heating temperature up to 1373 K and pyrolysis pressure up to 3 MPa. To avoid large temperature differences within the coal particles, the coal sample was heated from both the bottom and the top. The rate of heating to the final temperature and the residence time of the sample in this temperature were automatically controlled. A stream of 50 1 h21 helium was stabilized during the experiments. Within the whole experimental time of 90 s at various stages of pyrolysis the heating power was cut-off and the sample cooled by the flowing helium. This made possible the removal of the solid samples from the reactor for analysis. Data collected during the stage of linear heating at 5 or 2 s intervals and during isothermal holding at the final temperature at 15 s intervals, enabled the separation of the time and the temperature influences on the evaluated kinetic equations. The mass loss, composition and the porous structure of the samples removed from the reactor were then
Table 1 Characteristics of investigated coals Coal
W a (%)
V waf (%)
A wf (%)
C waf (%)
H waf (%)
N waf (%)
S a (%)
Real density (g cm23)
Initial porosity (vol%)
SIERSZA raw SIERSZA enriched NIWKA enriched
7.3 2.6 1.9
32.8 37.7 33.8
19.8 6.9 6.0
69.7 77.0 79.5
5.8 5.1 4.1
1.2 1.5 1.6
2.1 2.5 1.7
1.42 1.29 1.36
13.34 15.82 16.40
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287
Fig. 2. Coal sample mass loss as a function of pressure (enriched coal SIERSZA).
Fig. 1. Experimental unit.
measured. The structural investigations were carried out by means of a mercury porosimeter in which a pressure of 0.01 –200 MPa could be applied, allowing analyses of the pore size from 5 to 5000 nm. The correction for the modification of the coal and char structure by the mercury pressure was evaluated with the following parameters: coal compressibility, 1.6 £ 10210 m2 N21 [18]; mercury compressibility, 0.41 £ 10210 m2 N21 [18]; contact angle mercury/coal, 1358 [18]; mercury surface tension, 0.481 N m21 [19]. At each pyrolysis condition, three experiments were conducted and the final results of porosity and volatiles release were taken as an arithmetic mean value of three measurements. The error of the experimental method has been estimated as equal: for porosity 22.6% and for volatiles release 20.8%.
3. Results and discussion The influence of pressure on volatile mass release for enriched coal SIERSZA is presented in Fig. 2 for two rates of heating and two final heating temperatures. Within the analyzed temperature range for 0.1 MPa the amount of volatiles released is higher than that obtained under standard conditions (Table 1), the enhancement, however, does not exceed 30%. The previously described controversy concerning the amount of volatiles released during coal pyrolysis can be addressed for the lowest experimental pressure 0.1 MPa. The volatiles yield increase by 29% for the highest pyrolysis temperature 1373 K is caused partly by
fact that the V waf value in Table 1 was obtained for carbonization temperature 1123 K. For the lowest experimental temperature 1073 K, enhancement by 18% of volatiles mass was also found. However, the rate of heating applied during the experiments was not rapid, the yield of volatiles released from coal was higher than in standard carbonization conditions, even for the lowest temperature. The volatiles enhancement by about 30% at the highest temperature 1373 K is in close agreement with the results obtained by Loison and Chauvin [20] for a heating rate of 1500 K s21 to a final temperature of 1323 K. Because the last pyrolysis experiments [20] conducted on a metal gauze can certainly be qualified as rapid heating, and because the final pyrolysis temperatures differ only by 50 K, it may surely be concluded that fast pyrolysis does not enhance the release of volatiles by more than 30%. Also Anthony et al. [9], Suuberg et al. [21] and Cai et al. [8] reported volatiles yields increase only up to 25% during rapid pyrolysis (1000 K s21 to 1223 –1273 K) of small single layer coal samples in fixed bed reactors. What is more, Anthony et al. [9] found that within the heating rate 102 –104 K s21 the weight loss was independent of the heating rate. Determination of the devolatilization level from the change in ash level of the char, applied in most drop tube experiments during which volatiles enhancement by 80% was found [1 – 4], may be influenced by the gravitational separation of different density particles during the solids sampling process from the reactor. The agreement between the present fast and former rapid heating [8,9,20,28] experiments should not be regarded as incidental. In all cases the amount of volatiles was found on the basis of direct measurement of the sample mass loss, which is simple and accurate. It is very difficult to compare the influence of pressure on volatiles yield with the published data, mainly because of the different types of coal used but also the definition of the volatile mass release used by various authors (waf or analytical state) complicates the problem. Thus any comparison, such as presented in Fig. 3, can have only
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Fig. 3. Volatiles release as a function of pressure.
qualitative meaning. The observed strong influence of pressure on the volatiles release within the range 0.1– 1.5 MPa confirms the experiments of Anthony et al. [9] and Suuberg et al. [21] as well as the model of Niksa [12]. The escape of volatiles from a particle is a complex process involving volatiles formation and cracking to coke and light hydrocarbons within the particle and transport through a time-dependent pore structure. It has been previously observed [9,21] that the description of tar behaviour is the clue to the problem since tar is the main part of the volatiles. A strong drop in volatiles yield at moderate reactor pressure 0.1 –1.5 MPa most probably is an indication that pressure within the particle, determining the volatiles chemical reaction rate and transport, exceeds the reactor pressure by a comparable value. Tomeczek and Kowol [22] assuming a convective transport of volatiles through the porous structure found the pressure difference centre/surface within 1 mm particle size to be equal to about 0.1 MPa, confirming the last conclusion. This is the reason why in vacuum devolatilization the extent of the secondary reactions can be reduced due to enhanced volatiles transport, but only slightly above the 0.1 MPa reactor pressure conditions [9,21]. The total porosity variation of the carbonaceous particles for the same coal and under the same thermal conditions is presented in Fig. 4. Enhanced pressure decreases both the volatiles release as well the porosity variation. In all the examined cases an identical total pyrolysis time of 90 s was
Fig. 4. Porosity variation as a function of pressure (enriched coal SIERSZA).
used. The duration of the sample residence time at the final temperature is of particular importance when the experiments carried out under various conditions are compared, as shown in Table 2. During slow heating the final particle porosity is determined by the isothermal residence time at the final temperature. For fast heating, however, most of the porosity changes take place during the stage of intensive devolatilization but even in this case the isothermal period is important. Thus, in presenting porosity experiments it is necessary to inform not only about the rate of heating and the final temperature, but also about the exposure time at the final temperature. The comparison of the kinetics of volatiles release for the 0.1 and 1 MPa pyrolysis pressure is presented in Figs. 5 and 6. In both the cases the value of the maximum rate of volatiles release is similar, also the time position of this maximum can be considered independent of pressure. The rate of volatiles release has been described by the kinetic equation X Ej dmwaf waf ¼ k0j exp 2 Þ; ðmwaf 1j 2 m dt RT j¼1
ð1Þ
where the rate constant k0j ; Ej and mwaf 1j are given in Table 3 for the enriched coal SIERSZA. To describe well the experimental results it was necessary to assume up to six reactions at atmospheric pressure and five at 1.0 MPa. The curve calculated on the basis of the model for the SIERSZA
Table 2 Porosity as function of pyrolysis conditions (raw coal SIERSZA, 0.1 MPa) Final temperature (K)
Rate of heating
Porosity at time of reaching the final temperature (vol%)
Isothermal residence time in final temperature
Porosity after isothermal exposure (vol%)
673 873 1073 1373 1373 1373 1373
5 K min21 5 K min21 5 K min21 5 K min21 5 K min21 100 K s21 350 K s21
14.23 15.88 20.96 22.49 21.88 36.44 –
30 min 30 min 30 min 30 min 60 min 79 s 12 s
32.46 43.37 57.89 58.37 62.47 45.74 56.11
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289
Table 3 Kinetic constants in Eq. (1) for enriched coal SIERSZA Reaction j
1 2 3 4 5 6
Fig. 5. Measured and modelled rate of volatiles release (enriched coal SIERSZA).
in which the rate constants k0j ; Ej and 11j were evaluated on basis of the porosity measurements and are given in Table 4. For both analyzed pyrolysis pressures, five reactions had to
Fig. 6. Measured and modelled (kinetic constants Table 3) rate of volatiles release (enriched coal SIERSZA).
Ej (kJ mol21)
waf mwaf (%) 1j =m0
0.1 MPa
1 MPa
0.1 MPa
1 MPa
0.1 MPa
1 MPa
1 £ 105 1 £ 107 1 £ 107 1 £ 107 1 £ 108 1 £ 109
1 £ 105 1 £ 106 1 £ 107 1 £ 109 1 £ 1010 –
133 128 142 225 122 98
97 173 267 138 225 –
8.8 5.1 6.3 12.5 7.5 23.8
8.1 10.3 27.3 5.4 10.2 –
Table 4 Kinetic constants in Eq. (2) for raw (0.1 MPa) and enriched (1.0 MPa) coal SIERSZA Reaction j
coal given by Tomeczek and Kowol [22] for 0.1 MPa is presented in Fig. 5. The agreement between the present results and the modelling [20] is good, which certainly is due to similar coals being used in both the cases. Fig. 7 presents the comparison of the curve modelled by Eq. (1) with experimental points for the enriched coals SIERSZA and NIWKA. The agreement of the kinetic model with the experimental points is better for the SIERSZA than for NIWKA coal because the kinetic data in Table 3 were evaluated only on the basis of the first. The total porosity evolution has been modelled by a kinetic equation in which the values of the kinetic constants were assumed to be equal to that for volatiles release [13]. In the present study the porosity variation of the solid particles is described by X Ej d1 ¼ k0j exp 2 ð2Þ ð11j 2 1Þ; dt RT j¼1
k0j (s21)
1 2 3 4 5
k0j (s21)
Ej (kJ mol21)
11j (%)
0.1 MPa
1 MPa
0.1 MPa
1 MPa
0.1 MPa
1 MPa
1 £ 105 1 £ 107 1 £ 107 1 £ 108 1 £ 109
1 £ 105 1 £ 106 1 £ 107 1 £ 109 1 £ 109
131 127 145 105 306
132 201 257 169 182
9.2 2.4 7.8 3.5 41.2
11.1 13.1 32.8 3.5 2.9
be assumed in order to describe the experimental results. It can be seen that the kinetic constants given in Tables 3 and 4 are different, particularly for the last two reactions. Thus the assumption made by Simons [13] cannot be justified. Fig. 8 presents the modelling on the background of experiments for 1 MPa pyrolysis pressure. For both the coals the results are satisfactory. Comparison of Figs. 7 and 8 shows that the total particle porosity lags slightly behind the volatiles release, however, the values at maximum rate in both the cases are similar. Within the range of most intensive pyrolysis the delay times of porosity evolution for 1 MPa (Fig. 9) and for 0.1 MPa (Fig. 10) are similar. Most interesting, however, is the particle porous
Fig. 7. Measured and modelled rate of volatiles release.
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Fig. 11. Measured pore structure distribution (raw coal SIERSZA). Fig. 8. Measured and modelled rate of porosity variation.
groups and of the average pore size is presented in Fig. 13. The first size group undergoes drastic changes at short time and then remains relatively stable. The second pore size group increases gradually with time for 0.1 MPa pressure, while for 1 MPa this size pores remains almost stable. The largest evolution takes place in the pore size group 3, which for 0.1 MPa pressure developed gradually with time while for 1 MPa this pore size group after rapid increase at the beginning of pyrolysis continues to grow monotonically. The average pore size porosity increases almost linearly with time independent of pressure. In cases of heat and mass transfer modelling, knowledge of the pore size evolution is necessary. Simons [13] proposed a relation for the mean pore size time function Fig. 9. Measured and modelled rate of porosity variation and volatiles release (enriched coal SIERSZA).
structure. Figs. 11 and 12 present the porosity variation at 0.1 MPa and 1 MPa pyrolysis pressure for three pore size groups: 5 –30, 30– 300 and 300 –5000 nm. For both the values of the experimental pressure the total porosity evolution is the result of increase in only one size group 300 –5000 nm. The first group 5 – 30 nm almost totally collapsed. The time function of porosity of the three size
Fig. 10. Measured and modelled rate of porosity variation and volatiles release.
dp ðtÞ=dp ð0Þ ¼ ½1ðtÞ=1ð0Þ1=3 ;
ð3Þ
which, however, was not verified experimentally. Fig. 14 presents both the sides of this relation and additionally the function 1ðtÞ=1ð0Þ. It can be observed that the average pore diameter time function at atmospheric pressure is related quite well to the cubic root of porosity, while at 1 MPa pressure the average pore diameter increase is much stronger than the char porosity. Thus for 0.1 MPa pyrolysis pressure, the relation proposed by Simons must be taken
Fig. 12. Measured pore structure distribution (enriched coal SIERSZA).
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Fig. 13. Relationship for three pore size groups ð11;2;3 ðtÞ=11;2;3 ð0ÞÞ and average pore size ð1ðtÞ=1ð0ÞÞ for 20 K s21, 1073 K. A 11;2;3 ðtÞ=11;2;3 ð0Þ or 1ðtÞ=1ð0Þ; 0.1 MPa, raw coal SIERSZA, B 11;2;3 ðtÞ=11;2;3 ð0Þ or 1ðtÞ=1ð0Þ; 1.0 MPa enriched coal SIERSZA.
with caution. The cubic root relation at 0.1 MPa suggests a large contribution of spherical cavity pores within the char which were previously observed [23] for the parent coal.
4. Conclusions The kinetics of volatiles release and porosity evolution can be described by five reactions for which the activation energy depends on pressure. During the stage of most intensive devolatilization, the porosity evolution lags
behind the volatiles release. The delay time of porosity evolution was found to be independent of pyrolysis pressure. For the subbituminous coals analyzed, the total porosity evolution takes place within the macropores while the micropores almost collapse. At atmospheric pressure the average pore diameter time function related well to the cubic root of porosity, while at 1 MPa pressure the average pore diameter increase was much stronger. The average pore size porosity increased almost linearly with time for both 0.1 and 1 MPa pressure.
Fig. 14. Relationship between ½1ðtÞ=1ð0Þ1=3 ; 1ðtÞ=1ð0Þ and dp ðtÞ=dp ð0Þ for average pore size (20 K s21, 1073 K, raw coal SIERSZA 0.1 MPa and enriched coal SIERSZA 1 MPa). B dp ðtÞ=dp ð0Þ; p ½1ðtÞ=1ð0Þ1=3 ; A 1ðtÞ=1ð0Þ:
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The yield of volatiles during fast pyrolysis can be larger than that determined under standard conditions, but only by 30%. The final heating temperature and the residence time are the volatile yield controlling parameters during fast coal pyrolysis. Enhanced pressure decreased both the volatiles release and the total char porosity during coal pyrolysis. A strong drop of volatiles yield and porosity changes as pyrolysis pressure increased from 0.1 to 1.5 MPa was observed. The duration of the sample residence time at the final temperature is particularly important for particle porosity development during slow heating. For fast heating most of the porosity changes take place within the stage of intensive devolatilization but even in this case the isothermal period is important.
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[5] Heyd LE. MS Thesis. Department of Chemical Engineering, Princeton University; 1982. [6] Niksa S. 22nd Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1988. p. 105. [7] Niksa S, Heyd LE, Russel WB, Saville DA. 20th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1984. p. 1445. [8] Cai HY, Gu¨el AJ, Dugwell DR, Kandiyoti R. Fuel 1993;72:321. [9] Anthony DB, Howard JB, Hottel HC, Meissner HP. Fuel 1976;55:121. [10] Suuberg EM. ScD Thesis. MIT, Cambridge, MA; 1977. [11] Megaritis A, Zhuo YQ, Messenbo¨ck R, Dugwell DR, Kandiyoti R. Proceedings of the Ninth International Conference on Coal Science, Essen; 1997. p. 613. [12] Niksa S. Combust Flame 1995;100:384. [13] Simons GA. Combust Flame 1983;50:275. [14] Gavalas GR, Wilks KA. AICHE J 1980;26(2):201. [15] Singla PK, Miura S, Hudgima RR, Silveston PL. Fuel 1983;62:645. [16] Matsuo Y, Hayashi J-i, Kusakabe K, Morooka S. Proceedings of the Eighth International Conference on Coal Science, Oviedo. Amsterdam: Elsevier; 1995. [17] Weishauptova Z, Hedek J, Balek V, Sykorova I. Proceedings of the Eighth International Conference on Coal Science, Oviedo. Amsterdam: Elsevier; 1995. p. 933. [18] Spitzer Z. Powder Technol 1981;29:177. [19] Italiano P. Porosity—application analytical method. Milan: Carlo Erba Strumentazione Instruction; 1981. [20] Loison R, Chauvin R. Chim Ind 1964;91:269. [21] Suuberg EM, Peters WA, Howard JB. 17th Symposium (International) on Combustion. Pittsburgh, PA: The Combustion Institute; 1978. p. 117. [22] Tomeczek J, Kowol J. Can J Chem Engng 1991;69:286. [23] Tomeczek J, Mlonka J. Fuel 1998;77:1841.