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The influence of hydrogen donors on the pyrolysis of a bituminous coal as studied by in situ e.s.r. spectroscopy
Short Communications
reactivity of coal in the liquefaction reaction as the production of products solubles in benzene and n-hexane, the Amaga coal is more reactive than the Cerrejon coal.
ACKNOWLEDGEMENTS The authors wish to express their sincere thanks to Professor K. Ouchi (Hokkaido University, Japan) for his collaboration and
advice,
and
to the
University
of
Kyushu, Japan, for analyses of the coal samples. The financial support for this research was provided by the University of Antioquia and ICFES (Colombian Institute for the Promotion of the Higher Education). The Cerrejon and Amaga coal samples were obtained from Carbocol (Colombian Coal Company) and EDA (Antioquia State Company). REFERENCES 1 Mejia, 0. and Ribero, J. F. Energetica 1987, 1, 118
The influence of hydrogen donors on the pyrolysis coal as studied by in situ e.s.r. spectroscopy
2 Neavel, R. C. Fuel 1976,55,237 3 Mondragon,F., Makabe, M.,Itoh,H. and Ouchi, K. Fuel 1982,61, 392 4 Mondragon, F., Itoh, H. and Ouchi, K. Fue1 1984,63,668 5 Cusumano, J. A., Dalla Betta, R. A. and Levv, R. B. in ‘Catalysis in Coal Conversion’, Academic PreHs, 1978, p. 215 6 Neavel, R. C. in ‘Coal Science’, Volume I, (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, 1982, p. 5 7 Mondragon, F., Kamoshita, R., Katoh, T., Itoh, H. and Ouchi, K. Fuel 1984,63, 579 8 Derbyshire, F. and Stansberry, P. Fuel 1987,66, 174
of a bituminous
Timothy G. Fowlert, Rafael Kandiyoti” and Keith D. Bartle Department of Physical Chemistry, University of Leeds, Leeds, LS2 9JT, UK Department of Chemical Engineering and Chemical Technology, imperial College of Science and Technology, London, SW7 2BY, UK (Received 1 February 1988; revised 23 May 1988) l
The effect of hydrogen gas, a hydrogen donor solvent (tetralin) and a non-donor solvent (decane) on the pyrolysis (to 500°C) of a bituminous coal, before and after extraction with chloroform, has been studied by in situ e.s.r. in a flowing gas cell at atmospheric pressure. It was found that hydrogen gas at 1 bar had an insignificant effect on the course of the reaction, as determined by free radical population measurements, compared with nitrogen gas. In contrast, both tetralin and decane change the free radical populations developed during pyrolysis, and the extent of the induced change varies upon chloroform extraction of the coal. These results are discussed with reference to current coal liquefaction models, and are interpreted in terms of the chemical and physical interactions of the solvent with the coal. (Keywords: bituminous coal; pyrolysis; electron spin resonance)
Recent work’-’ has used in situ e.s.r. to study coal pyrolysis using a variety of reactor geometries to determine the influence of pyrolysis-derived volatiles on the reaction pathways. In contrast to previous work’-’ ‘, which was performed in sealed tubes, these experiments have used both sealed tube cells2~’ and open cells. In the open cells, the sample is either continuously swept with nitrogen gas’-‘, or is continuously evacuated by a rotary vacuum pump 3x5. Measurements of in sifu e.s.r. spin populations generally assume a Curie Law l/T temperature correction in the calculation. On testing this assumption, it was found that the temperature dependence from samples in sealed tubes conformed poorly to the Curie Law, but a change to an open reactor geometry (flow or vacuum) yielded results which were more consistent with the (Curie Law) assumptions made in calculating the spin t Present address: Research Station, B91 2JW, UK
British Gas, Midlands Wharf Lane, Solihull,
001~2361/88/121711~3$3.00 (i’,1988 Butterworth & Co. (Publishers) Ltd.
population2. Experiments with a range of coals confirmed this observation”. This result was interpreted to mean that pyrolysis volatiles are in equilibrium with stable free radicals in the char in sealed tubes, and that the position of this equilibrium varies with temperature, thus affecting the spin population5. In situ e.s.r. has also been used to study the pyrolysis of coal in the presence of hydrogen donor (H-donor) solvents. One generally observed result was that the presence of an H-donor during pyrolysis depresses the free radical concentration relative to coal similarly pyrolysed alone6-9*’ 3 This result has been qualitativeli interpreted to mean that Hdonors make more hydrogen available to newly created free radicals, formed by thermolysis, and increase the transport of native hydrogen in the coal to the bondbreaking sites (H-shuttling14). Such an interpretation predicts an increased conversion to low molecular weight products on pyrolysis in the presence of an H-donor, due to the more rapid transfer of stabilizing hydrogen atoms to
reactive coal fragments formed by thermolysis thus preventing their repolymerization, as is observed6*9~10~1 5. In view of the uncertainty regarding the calculation of the spin population from closed tube systems expressed above, it was decided to perform a limited number of experiments in open cells with a variety of chemical additives, to determine their influence on the pyrolysis pathway at atmospheric pressure as measured by in situ e.s.r. EXPERIMENTAL Linbycoal (C83.O”/;H 5.5%,08.7;& N 1.9% dmmf) was used as received from the British Coal Bank. The experimental method and calculation techniques were largely as described elsewhere1*2, as were the calculations of spin concentrations (free radicals per gram at room temperature) and spin populations (S free radicals per gram of initial sample mass at elevated temperature; this definition allows for the escape of volatiles from the e.s.r. active zone).
FUEL,
1988,
Vol 67, December
1711
Short Communications Table 1 Effect of atmosphere and solvent on the spin population of Linby and chloroformextracted Linby at room temperature. While the differences cited in this table are at the limit of experimental significance, the trends expressed are reproducible, even if the exact numbers are not s x 10-*9/g Atmosphere
Air
Time
0
5
10
15
20
Linby
1.07
1.14
_
_
_
Linby Linby Extd. Linby
1.07 1.07 0.98
1.18 1.17 0.99
1.17 1.18 0.99
1.16 1.14 _
1.15 1.14 _
Decane Tetralin
Extd. Linby Extd. Linby
0.98 0.98
0.99 1.00
1.00 1.02
1.01 0.99
1.00 0.99
Decane Tetralin
Sample
Nitrogen
Experiments were performed with nitrogen or hydrogen (at nominal 1 bar pressure) or with nitrogen supplemented with a drip feed of tetralin or n-decane (BDH, > 98 %) from a syringe at a flow rate of about 0.01cm3 min-‘. In each case the gas sweep velocity was measured with a calibrated rotameter to be less than 0.15 m s-l at room temperature. Tetralin is often used as an H-donor6P1 5 and ndecane was chosen as a control because of its similar physical properties. The exhaust from the sweep gas stream was routed through a bunsen flame to incinerate the unused hydrogen and other noncondensible volatiles. The ‘vacuum’ ce113v5(in flowing gas mode) was used for the current series of experiments because in the flow cell’ the sample sweep gas flows through the main gas heater prior to sweeping the sample bed. The reverse is true of the vacuum cell, and therefore errors due to decomposition of solvent in
Nitrogen
‘0
10
20
30
40
50
60
Time (min)
Figure 1 Temperature
and spin population shown as functions of time. Vertical division on the temperature line are at 104PC intervals. Error bars indicated are &standard deviation/n’.‘, where n is the number of independent experiments. a 0, Linby+nitrogen (2 runs); n , Linby + hydrogen (2 runs); b 0, Linby+nitrogen; A, Linby+nitrogen+ tetralin (4 runs); c A, Linby+nitrogen+ tetralin; +, Linby + nitrogen + decane (2 runs)
1712
FUEL,
1988,
Vol 67, December
Solvent
the gas heater (at temperatures greater than SOO’C) prior to contact with the sample were prevented. The disadvantages of this arrangement are that, first, the sweep stream is not preheated and therefore rapid sweep velocities are not accessible3p4 and, second, the solvent drips down the temperature control thermocouple, which on vaporiza;ion, leads to rapid fluctuations (by up to YC) in recorded temperature. While this made the experiment less easy to perform, it is not considered that temperature control problems affected the quality of the results, as the mean temperature ramp applied (1O’C min-’ to 500°C with a hold period of 12 min at peak temperature) was maintained. Some systematic differences in the S profile for Linby measured in the flow cell and the vacuum cell under nominally identical conditions were noted (further details will be published later); these were attributed to detailed differences in the design of the cells. This observation reiterates the principle that pyrolysis is a geometrydependent process and so comparisons between different experimental rigs should only be made with extreme care. RESULTS
:-
fsolvent
and the first minimum in S (at about 300°C) respectively’), but in region 3 (beyond the first minimum in S (above about 300°C)‘) tetralin shows a slight tendency to depress S more efficiently than decane. Figure 2 shows the set of curves equivalent to Figure 1 but for chloroform-extracted Linby coal’. The temperature dependence of the signal after these runs was also checked and found to be as consistent with the Curie Law assumptions as was that for unextracted Linby. Figure 2 confirms the inactivity of hydrogen gas seen in Figure 1 and also shows that the depressing effect of the solvent on S in regions 1 and 2, seen in Figure 1, is almost completely absent for the extracted coal. Moreover, Figure 2 shows a second line obtained from chloroform-extracted Linby plus decane. The two sets of results with decane were obtained from live apparently identical experiments, which fell into two groups as shown. No satisfactory explanation is offered for this behaviour, although it is noted that in Figure I the error limits of the decane line are wider than for any of the other curves. The explanation may lie in the stepwise addition of solvent to the hot sample which, for some reason, influences the effect of decane more than that of tetralin on the results. Therefore it is not clear from Figure 2, how extracted Linby plus decane behaves in region 3. The processes that produce the three region behaviour seen in Figures J and 2 have been discussed previously, as have
AND DISCUSSION
Figure 1 shows the S profile obtained when Linby is pyrolysed in pure nitrogen, of pure hydrogen, or in nitrogen plus tetralin or decane. The temperature dependence of the e.s.r. signal was checked after each run and found to be linear with extrapolated Weiss constants of less than 100 K, indicating reasonable agreement with the Curie Law and thus validating the calculated values of S (Ref. 2). In agreement with weight loss and tar yield data 16, in situ e.s.r. indicates that pyrolysis in a hydrogen atmosphere at 1 bar has an insignificant effect on the course of the reaction up to 500°C. In contrast, both decane and tetralin depress the value of S measured at room temperature (Table I), and right across the temperature range up to 500°C. Decane and tetralin seem to behave identically in regions 1 and 2 (defined as up to the first maximum in S (at about 200°C) and between the first maximum
0
10
20
30
40
50
60
Time (mid
Figure 2
Temperature and spin population shown as functions of time. Vertical divisions on the temperature line are at 100°C intervals. Error bars indicated are &standard deviation/P, where n is the number of independent experiments. a, 0, Extracted Linby+nitrogen (2 runs); n , extracted Linby+ hydrogen (2 runs); b, 0, extracted Linby+nitrogen; A, extracted Linby fnitrogen+ tetralin (2 runs); c, A, extracted Linby+nitrogen+ tetralin; --t, extracted Linby + nitrogen + decane (3 runs); --C-, extracted Linby +nitrogen -!-decane (2 runs)
Short Communications the differences between Linby and chloroform-extracted Linby’. Briefly, the rise in S in region 1 is associated with and other desorption of oxygen adsorbates which unmasks previously unobservable radicals. The fall in S in region 2 is ascribed to thermally activated mobility and consequent radical recombination, and region 3 is the result of the onset of bond thermolysis. The differences observed by in situ e.s.r. between Linby and chloroform-extracted Linby on pyrolysis were confined mainly to region 2. Chloroform extraction removes a fraction of the total unbound low molecular weight material, including a low concentration of free radicals”, from the pores of the coal. This results in a pore structure that is less constricted and so reduces the activation energy for diffusion-controlled processes for those mobile species that remain. Thus diffusion-controlled recombination reactions proceed at lower temperatures after extraction. (It has previously been deduced that the pore structure ofcoals of higher rank than lignite is unaffected by drying’ 8, and the effect on the pore structure of chloroform extraction is probably no more severe.) On the basis of fitting coal conversion yields as a function of time to model free radical reactions the free radical mechanism for liquefaction in the presence of a donor solvent (DH,) has five schematic been reduced to equations’ 5 : Coal+ZR. R.+DH,+R-H+DH. R. + Coal-H *Coal. + R-H R.+DH.-+R-H +D R. + Coal-H.+ R-H + Coal
(1) (2) (3) (4) (5)
where R. is a free radical intermediate. The net result of these equations is the stabilization of coal fragments, formed in Equation (l), and the dehydrogenation of the solvent and/or the hydrogen rich portions of the coal. It is not clear what relation the radicals R., DH., Coal. and Coal-H. bear to the radicals observed in in situ e.s.r. studies. Indeed, it has previously been argued that reactive intermediates are unlikely to be observed e.s.r.‘,3-s equipment, with in situ although other workers have claimed such an observation for one coa17, which however was not repeatable with other samples”. It is considered that Equations (l)-(5) above are an oversimplification of a process that involves reactive, probably e.s.r.-silent, radical intermediates, stable e.s.r.-active product radicals and an interplay between species that are mobile and those that are bound to the coal macromolecule. These ideas
will be more formally developed in a future publication. In keeping with the conclusions of other workerP, it seems plausible that the presence of tetralin depresses the value of S in region 3, relative to the pyrolysis of coal alone, via a hydrogen transfer mechanism similar to the one given above. However, it is noted that Equations (lH5) can only account for ‘chemical’ behaviour in region 3, as significant bond thermolysis is unlikely below 300”C6.20, yet the presence of solvent influences the results at low temperatures, as shown in Figures 1 and 2 and Table 1. It is suggested that this is due to a ‘physical’ solvation mechanism. It is known that tetralin swells coal signiand, while decane is not ficantly’i expected to swell coal to the same extent, it is likely that even a slight swelling or relaxing of the coal pore structure would reduce the activation energy for diffusion and so allow radical recombination reactions, usually characteristic of region 2, to proceed at lower temperatures.
solvent, the influence of molecular hydrogen (at 1 bar) on the pyrolytic processes of coal and extracted coal is negligible according to Figures 1 and 2. An explanation for this lies in the relatively high dissociation energy of molecular hydrogen compared with bonds expected in the coal structure6.“.
ACKNOWLEDGEMENTS The authors would like to thank British Coal for the supply of samples and SERC for financial support under research grant GR/D/03581.
REFERENCES
(Radical recombination processes are most affected by this because they involve
diffusion of relatively large molecules compared with the oxygen transport usually characteristic of region 1.) This solvent effect in regions 1 and 2 is not seen for extracted Linby (Table 1 and Figure 2) because in this case the coal has been previously equilibrated at about 60°C in chloroform, and not only has a fraction of the mobile species been removed, but also some of the low temperature radical recombination reactions observed in Figure 1 have reached completion. It is apparent that chloroform extraction does not affect coal in the same way, or to the same extent, as pyrolysis in the presence of tetralin or decane since, for both tetralin and decane, the value of S for Linby plus solvent is less than that for extracted Linby plus solvent over the entire temperature range, excepting one or two points near room temperature. That is, the effects of chloroform extraction and added tetralin or decane during pyrolysis are neither additive, as the value of S for extracted coal plus solvent would then be lower than that for unextracted coal plus solvent, nor are they equivalent, as then the value of S for extracted coal plus solvent would be the same as that for unextracted coal plus solvent. This could be because different solvents will interact with the coal in different ways, and the order in which they are contacted may be significant. Whatever the explanation, it is clear that there are important interactions in coal/solvent systems at pre-pyrolysis temperatures which can be observed by e.s.r. In contrast to the action of an H-donor
6
I 8
9 10 I1 12
13 14 15
16
17
18
19
20 21
FUEL,
Fowler, T. G., Bartle, K. D. and Kandiyoti, R. Furl 1987, 66, 1407 Fowler, T. G., Bartle, K. D. and Kandiyoti, R. Curhon 1987, 25, 709 Gonenc, 2. S., Fowler, T. G., Kandiyoti, R. and Bartle, K. D. Fuel 1988, 67, 848 Fowler, T. G., Bartle, K. D. and Kandiyoti, R. Fuel 1988,67, 173 Fowler, T. G., Kandiyoti, R., Bartle. K. D. and Tavlor. N. in ‘Proc. 1987 Int. Conf. on 1 Coal Science‘, Elsevier, Amsterdam, 1987, p. 617 Petrakis, L. and Grandy, D. W. in ‘Free Radicals in Coals and Synthetic Fuels’, Elsevier, Amsterdam, 1983 Sprecher, R. F. and Retcofsky, H. L. Fuel 1983, 62, 473 Goldberg, I. B., McKinney, T. M. and Ratto, J. J. in ‘Proc. 1983 Int. Conf. Coal Science’, International Energy Agency, Pittsburgh, USA, p. 142 Stenberg, V. I., Jones, M. B. and Suwamasan, N. J. Fuel 1985.64, 470 Rudnick, L. R. and Tueting, D. Fuel 1984, 63, 153 Sakawa, M., Uno, T. and Hara, Y. Fuel 1983,62, 57 1 Fowler, T. G., Kandiyoti, R., Bartle, K. D. and Snape, C. E. Carbon, submitted for publication Yokono, T., Kohno, T. and Sanada, Y. Fuel 1985,64,411 Neavel, R. C. Fuel 1976,55,237 Bockrath, B. C. in ‘Coal Science Volume 2’ (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, 1983, p. 65 Howard, J. B. in ‘Chemistry of Coal Utilization: Supplementary Volume 2’ (Ed. M. M. Elliott), J. Wiley/Interscience, 198 I. p. 753 Neuburg, H. J., Kandiyoti, R., O’Brien, R. J., Fowler,T. G. and Bartle, K. D. Fuel 1987,66,486 Gorbaty, M. L., Mraw, S. C., Gethner, J. S. and Brenner, D. Fuel Proc. Tech. 1986, 12,31 Sprecher, R. F., Lett, R. G. and Retcofsky, H. L. in ‘Proc. Int. Conf. Coal Science’ 1983, IEA, Pittsburgh, USA, p. 545 Gavalas, G. R. in ‘Coal Pyrolysis’, Elsevier, Amsterdam. 1982 Larsen,J.W.,Green,T.K.andChiri,I.in ‘Proc. Int. Conf. Coal Science’, 1983, IEA, Pittsburgh, USA, p. 277
1988,
Vol 67, December
1713
reactivity of coal in the liquefaction reaction as the production of products solubles in benzene and n-hexane, the Amaga coal is more reactive than the Cerrejon coal.
ACKNOWLEDGEMENTS The authors wish to express their sincere thanks to Professor K. Ouchi (Hokkaido University, Japan) for his collaboration and
advice,
and
to the
University
of
Kyushu, Japan, for analyses of the coal samples. The financial support for this research was provided by the University of Antioquia and ICFES (Colombian Institute for the Promotion of the Higher Education). The Cerrejon and Amaga coal samples were obtained from Carbocol (Colombian Coal Company) and EDA (Antioquia State Company). REFERENCES 1 Mejia, 0. and Ribero, J. F. Energetica 1987, 1, 118
The influence of hydrogen donors on the pyrolysis coal as studied by in situ e.s.r. spectroscopy
2 Neavel, R. C. Fuel 1976,55,237 3 Mondragon,F., Makabe, M.,Itoh,H. and Ouchi, K. Fuel 1982,61, 392 4 Mondragon, F., Itoh, H. and Ouchi, K. Fue1 1984,63,668 5 Cusumano, J. A., Dalla Betta, R. A. and Levv, R. B. in ‘Catalysis in Coal Conversion’, Academic PreHs, 1978, p. 215 6 Neavel, R. C. in ‘Coal Science’, Volume I, (Eds. M. L. Gorbaty, J. W. Larsen and I. Wender), Academic Press, 1982, p. 5 7 Mondragon, F., Kamoshita, R., Katoh, T., Itoh, H. and Ouchi, K. Fuel 1984,63, 579 8 Derbyshire, F. and Stansberry, P. Fuel 1987,66, 174
of a bituminous
Timothy G. Fowlert, Rafael Kandiyoti” and Keith D. Bartle Department of Physical Chemistry, University of Leeds, Leeds, LS2 9JT, UK Department of Chemical Engineering and Chemical Technology, imperial College of Science and Technology, London, SW7 2BY, UK (Received 1 February 1988; revised 23 May 1988) l
The effect of hydrogen gas, a hydrogen donor solvent (tetralin) and a non-donor solvent (decane) on the pyrolysis (to 500°C) of a bituminous coal, before and after extraction with chloroform, has been studied by in situ e.s.r. in a flowing gas cell at atmospheric pressure. It was found that hydrogen gas at 1 bar had an insignificant effect on the course of the reaction, as determined by free radical population measurements, compared with nitrogen gas. In contrast, both tetralin and decane change the free radical populations developed during pyrolysis, and the extent of the induced change varies upon chloroform extraction of the coal. These results are discussed with reference to current coal liquefaction models, and are interpreted in terms of the chemical and physical interactions of the solvent with the coal. (Keywords: bituminous coal; pyrolysis; electron spin resonance)
Recent work’-’ has used in situ e.s.r. to study coal pyrolysis using a variety of reactor geometries to determine the influence of pyrolysis-derived volatiles on the reaction pathways. In contrast to previous work’-’ ‘, which was performed in sealed tubes, these experiments have used both sealed tube cells2~’ and open cells. In the open cells, the sample is either continuously swept with nitrogen gas’-‘, or is continuously evacuated by a rotary vacuum pump 3x5. Measurements of in sifu e.s.r. spin populations generally assume a Curie Law l/T temperature correction in the calculation. On testing this assumption, it was found that the temperature dependence from samples in sealed tubes conformed poorly to the Curie Law, but a change to an open reactor geometry (flow or vacuum) yielded results which were more consistent with the (Curie Law) assumptions made in calculating the spin t Present address: Research Station, B91 2JW, UK
British Gas, Midlands Wharf Lane, Solihull,
001~2361/88/121711~3$3.00 (i’,1988 Butterworth & Co. (Publishers) Ltd.
population2. Experiments with a range of coals confirmed this observation”. This result was interpreted to mean that pyrolysis volatiles are in equilibrium with stable free radicals in the char in sealed tubes, and that the position of this equilibrium varies with temperature, thus affecting the spin population5. In situ e.s.r. has also been used to study the pyrolysis of coal in the presence of hydrogen donor (H-donor) solvents. One generally observed result was that the presence of an H-donor during pyrolysis depresses the free radical concentration relative to coal similarly pyrolysed alone6-9*’ 3 This result has been qualitativeli interpreted to mean that Hdonors make more hydrogen available to newly created free radicals, formed by thermolysis, and increase the transport of native hydrogen in the coal to the bondbreaking sites (H-shuttling14). Such an interpretation predicts an increased conversion to low molecular weight products on pyrolysis in the presence of an H-donor, due to the more rapid transfer of stabilizing hydrogen atoms to
reactive coal fragments formed by thermolysis thus preventing their repolymerization, as is observed6*9~10~1 5. In view of the uncertainty regarding the calculation of the spin population from closed tube systems expressed above, it was decided to perform a limited number of experiments in open cells with a variety of chemical additives, to determine their influence on the pyrolysis pathway at atmospheric pressure as measured by in situ e.s.r. EXPERIMENTAL Linbycoal (C83.O”/;H 5.5%,08.7;& N 1.9% dmmf) was used as received from the British Coal Bank. The experimental method and calculation techniques were largely as described elsewhere1*2, as were the calculations of spin concentrations (free radicals per gram at room temperature) and spin populations (S free radicals per gram of initial sample mass at elevated temperature; this definition allows for the escape of volatiles from the e.s.r. active zone).
FUEL,
1988,
Vol 67, December
1711
Short Communications Table 1 Effect of atmosphere and solvent on the spin population of Linby and chloroformextracted Linby at room temperature. While the differences cited in this table are at the limit of experimental significance, the trends expressed are reproducible, even if the exact numbers are not s x 10-*9/g Atmosphere
Air
Time
0
5
10
15
20
Linby
1.07
1.14
_
_
_
Linby Linby Extd. Linby
1.07 1.07 0.98
1.18 1.17 0.99
1.17 1.18 0.99
1.16 1.14 _
1.15 1.14 _
Decane Tetralin
Extd. Linby Extd. Linby
0.98 0.98
0.99 1.00
1.00 1.02
1.01 0.99
1.00 0.99
Decane Tetralin
Sample
Nitrogen
Experiments were performed with nitrogen or hydrogen (at nominal 1 bar pressure) or with nitrogen supplemented with a drip feed of tetralin or n-decane (BDH, > 98 %) from a syringe at a flow rate of about 0.01cm3 min-‘. In each case the gas sweep velocity was measured with a calibrated rotameter to be less than 0.15 m s-l at room temperature. Tetralin is often used as an H-donor6P1 5 and ndecane was chosen as a control because of its similar physical properties. The exhaust from the sweep gas stream was routed through a bunsen flame to incinerate the unused hydrogen and other noncondensible volatiles. The ‘vacuum’ ce113v5(in flowing gas mode) was used for the current series of experiments because in the flow cell’ the sample sweep gas flows through the main gas heater prior to sweeping the sample bed. The reverse is true of the vacuum cell, and therefore errors due to decomposition of solvent in
Nitrogen
‘0
10
20
30
40
50
60
Time (min)
Figure 1 Temperature
and spin population shown as functions of time. Vertical division on the temperature line are at 104PC intervals. Error bars indicated are &standard deviation/n’.‘, where n is the number of independent experiments. a 0, Linby+nitrogen (2 runs); n , Linby + hydrogen (2 runs); b 0, Linby+nitrogen; A, Linby+nitrogen+ tetralin (4 runs); c A, Linby+nitrogen+ tetralin; +, Linby + nitrogen + decane (2 runs)
1712
FUEL,
1988,
Vol 67, December
Solvent
the gas heater (at temperatures greater than SOO’C) prior to contact with the sample were prevented. The disadvantages of this arrangement are that, first, the sweep stream is not preheated and therefore rapid sweep velocities are not accessible3p4 and, second, the solvent drips down the temperature control thermocouple, which on vaporiza;ion, leads to rapid fluctuations (by up to YC) in recorded temperature. While this made the experiment less easy to perform, it is not considered that temperature control problems affected the quality of the results, as the mean temperature ramp applied (1O’C min-’ to 500°C with a hold period of 12 min at peak temperature) was maintained. Some systematic differences in the S profile for Linby measured in the flow cell and the vacuum cell under nominally identical conditions were noted (further details will be published later); these were attributed to detailed differences in the design of the cells. This observation reiterates the principle that pyrolysis is a geometrydependent process and so comparisons between different experimental rigs should only be made with extreme care. RESULTS
:-
fsolvent
and the first minimum in S (at about 300°C) respectively’), but in region 3 (beyond the first minimum in S (above about 300°C)‘) tetralin shows a slight tendency to depress S more efficiently than decane. Figure 2 shows the set of curves equivalent to Figure 1 but for chloroform-extracted Linby coal’. The temperature dependence of the signal after these runs was also checked and found to be as consistent with the Curie Law assumptions as was that for unextracted Linby. Figure 2 confirms the inactivity of hydrogen gas seen in Figure 1 and also shows that the depressing effect of the solvent on S in regions 1 and 2, seen in Figure 1, is almost completely absent for the extracted coal. Moreover, Figure 2 shows a second line obtained from chloroform-extracted Linby plus decane. The two sets of results with decane were obtained from live apparently identical experiments, which fell into two groups as shown. No satisfactory explanation is offered for this behaviour, although it is noted that in Figure I the error limits of the decane line are wider than for any of the other curves. The explanation may lie in the stepwise addition of solvent to the hot sample which, for some reason, influences the effect of decane more than that of tetralin on the results. Therefore it is not clear from Figure 2, how extracted Linby plus decane behaves in region 3. The processes that produce the three region behaviour seen in Figures J and 2 have been discussed previously, as have
AND DISCUSSION
Figure 1 shows the S profile obtained when Linby is pyrolysed in pure nitrogen, of pure hydrogen, or in nitrogen plus tetralin or decane. The temperature dependence of the e.s.r. signal was checked after each run and found to be linear with extrapolated Weiss constants of less than 100 K, indicating reasonable agreement with the Curie Law and thus validating the calculated values of S (Ref. 2). In agreement with weight loss and tar yield data 16, in situ e.s.r. indicates that pyrolysis in a hydrogen atmosphere at 1 bar has an insignificant effect on the course of the reaction up to 500°C. In contrast, both decane and tetralin depress the value of S measured at room temperature (Table I), and right across the temperature range up to 500°C. Decane and tetralin seem to behave identically in regions 1 and 2 (defined as up to the first maximum in S (at about 200°C) and between the first maximum
0
10
20
30
40
50
60
Time (mid
Figure 2
Temperature and spin population shown as functions of time. Vertical divisions on the temperature line are at 100°C intervals. Error bars indicated are &standard deviation/P, where n is the number of independent experiments. a, 0, Extracted Linby+nitrogen (2 runs); n , extracted Linby+ hydrogen (2 runs); b, 0, extracted Linby+nitrogen; A, extracted Linby fnitrogen+ tetralin (2 runs); c, A, extracted Linby+nitrogen+ tetralin; --t, extracted Linby + nitrogen + decane (3 runs); --C-, extracted Linby +nitrogen -!-decane (2 runs)
Short Communications the differences between Linby and chloroform-extracted Linby’. Briefly, the rise in S in region 1 is associated with and other desorption of oxygen adsorbates which unmasks previously unobservable radicals. The fall in S in region 2 is ascribed to thermally activated mobility and consequent radical recombination, and region 3 is the result of the onset of bond thermolysis. The differences observed by in situ e.s.r. between Linby and chloroform-extracted Linby on pyrolysis were confined mainly to region 2. Chloroform extraction removes a fraction of the total unbound low molecular weight material, including a low concentration of free radicals”, from the pores of the coal. This results in a pore structure that is less constricted and so reduces the activation energy for diffusion-controlled processes for those mobile species that remain. Thus diffusion-controlled recombination reactions proceed at lower temperatures after extraction. (It has previously been deduced that the pore structure ofcoals of higher rank than lignite is unaffected by drying’ 8, and the effect on the pore structure of chloroform extraction is probably no more severe.) On the basis of fitting coal conversion yields as a function of time to model free radical reactions the free radical mechanism for liquefaction in the presence of a donor solvent (DH,) has five schematic been reduced to equations’ 5 : Coal+ZR. R.+DH,+R-H+DH. R. + Coal-H *Coal. + R-H R.+DH.-+R-H +D R. + Coal-H.+ R-H + Coal
(1) (2) (3) (4) (5)
where R. is a free radical intermediate. The net result of these equations is the stabilization of coal fragments, formed in Equation (l), and the dehydrogenation of the solvent and/or the hydrogen rich portions of the coal. It is not clear what relation the radicals R., DH., Coal. and Coal-H. bear to the radicals observed in in situ e.s.r. studies. Indeed, it has previously been argued that reactive intermediates are unlikely to be observed e.s.r.‘,3-s equipment, with in situ although other workers have claimed such an observation for one coa17, which however was not repeatable with other samples”. It is considered that Equations (l)-(5) above are an oversimplification of a process that involves reactive, probably e.s.r.-silent, radical intermediates, stable e.s.r.-active product radicals and an interplay between species that are mobile and those that are bound to the coal macromolecule. These ideas
will be more formally developed in a future publication. In keeping with the conclusions of other workerP, it seems plausible that the presence of tetralin depresses the value of S in region 3, relative to the pyrolysis of coal alone, via a hydrogen transfer mechanism similar to the one given above. However, it is noted that Equations (lH5) can only account for ‘chemical’ behaviour in region 3, as significant bond thermolysis is unlikely below 300”C6.20, yet the presence of solvent influences the results at low temperatures, as shown in Figures 1 and 2 and Table 1. It is suggested that this is due to a ‘physical’ solvation mechanism. It is known that tetralin swells coal signiand, while decane is not ficantly’i expected to swell coal to the same extent, it is likely that even a slight swelling or relaxing of the coal pore structure would reduce the activation energy for diffusion and so allow radical recombination reactions, usually characteristic of region 2, to proceed at lower temperatures.
solvent, the influence of molecular hydrogen (at 1 bar) on the pyrolytic processes of coal and extracted coal is negligible according to Figures 1 and 2. An explanation for this lies in the relatively high dissociation energy of molecular hydrogen compared with bonds expected in the coal structure6.“.
ACKNOWLEDGEMENTS The authors would like to thank British Coal for the supply of samples and SERC for financial support under research grant GR/D/03581.
REFERENCES
(Radical recombination processes are most affected by this because they involve
diffusion of relatively large molecules compared with the oxygen transport usually characteristic of region 1.) This solvent effect in regions 1 and 2 is not seen for extracted Linby (Table 1 and Figure 2) because in this case the coal has been previously equilibrated at about 60°C in chloroform, and not only has a fraction of the mobile species been removed, but also some of the low temperature radical recombination reactions observed in Figure 1 have reached completion. It is apparent that chloroform extraction does not affect coal in the same way, or to the same extent, as pyrolysis in the presence of tetralin or decane since, for both tetralin and decane, the value of S for Linby plus solvent is less than that for extracted Linby plus solvent over the entire temperature range, excepting one or two points near room temperature. That is, the effects of chloroform extraction and added tetralin or decane during pyrolysis are neither additive, as the value of S for extracted coal plus solvent would then be lower than that for unextracted coal plus solvent, nor are they equivalent, as then the value of S for extracted coal plus solvent would be the same as that for unextracted coal plus solvent. This could be because different solvents will interact with the coal in different ways, and the order in which they are contacted may be significant. Whatever the explanation, it is clear that there are important interactions in coal/solvent systems at pre-pyrolysis temperatures which can be observed by e.s.r. In contrast to the action of an H-donor
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FUEL,
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