436
Energy & Fuels 1991,5, 436-440
polycyclic aromatic hydrocarbons with low and short side chains. This tar may have a high mutagenic activity. Acknowledgment. Thanks are due to Professor s. Kaliaguine for permission to use the FTIR spectrometer. The assistance of Dr. J. L. Grandmaison to record FTIR spectra is also gratefully acknowledged. The gasification
tar sample was kindly provided by Biosyn (Montreal, P.Q.). The collaboration of Mr. Guy Drouin from Biothermica International Inc. (Montreal) is also acknowledged. This study has been by Mines and Canada and Energie et Ressources Qu6bec.
Ultrarapid Flashlamp Pyrolysis: Thermal versus Photochemical Reaction Pathways John H. Penn* and Walter H. Owens Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045
Lawrence J. Shadle U S . Department of Energy, Morgantown Energy Technology Center, Morgantown, West Virginia 26505 Received October 22, 1990. Revised Manuscript Received December 18, 1990
A ;enon flashlamp has been used to rapidly heat organic substrates coated onto graphite particles by conversion of photochemical energy into thermal energy within the graphite particles, resulting in heating rates of 105"C/s (i-e.,ultrarapid pyrolysis). The compounds chosen for ultrarapid pyrolysis have been carefully selected to verify whether the reported reactions are due to photochemical or thermal activation. Bibenzyl yields primarily acetylene when subjected to flashlamp conditions. In contrast, traditional thermal or photochemical excitation yields products which may be rationalized by cleavage of the central C-C bond to yield benzyl radicals. 1,6-Diphenylhexanealso yields acetylene with a significant amount of ethylene when exposed to ultrarapid heating conditions. With traditional heating and photochemical reaction conditions, no products can be found. 1,4-Dibenzoylbutaneyields the normal Norrish type I1 cleavage products upon photoexcitation and no products upon thermal excitation at 650 OC. In contrast, ultrarapid pyrolysis yields acetylene and a small amount of benzaldehyde. Taken together these results indicate that photochemical reactions cannot be responsible for the observed reactions. Since the ultrarapid pyrolysis products differ from those observed with traditional heating techniques, the ultrarapid pyrolysis products are attributed to higher temperature (>lo00 "C) thermal activation.
Introduction A variety of studies, in a laboratory effort to emulate high heating rate industrial process conditions, such as fluidized bed reactors or entrained flow reactors, have shown that changes in the heating rate of fossil fuel substrates can dramatically change the relative quantities of the products formed in pyrolysis reactions.' These laboratory techniques for achieving high heating rates include wire-grid pyrolysis,2a shock tube reactions,2b flashlamp pyrolysis: laser pyrolysis,'*s fluidized sand bed pyrolysis,S (1)Solomon, P. R.;Hamblen, D. G. In Chemistry of Coal Conversion; SchJosberg, R. H., Ed.;Plenum Press: New York, 1986; Chapter 5. (2)(a) Cliff, D. I.; Doolan, K. R.; Mackie, J. C.; Tyler. R. J. Fuel 1984. 63,394. (b) Tyler, R. J. Fuel 1980,69,218. (3)(a) Freihaut, J. D.; Proecia, W. M. Energy Fuels 1989,3,625.(b) Freihaut, J. D.; Zabielski, M. F.; Seery, D. J. Ninteenth Symposium (Znternutional)on Combuntion;The Combustion Institute: Pithburgh, 1982;p 1159. (c) Freihaut, J. D.; Proecia, W. M.; Seery, D. J. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1987,32(3),129. (4) Phuoc, T. X.; Maloney, D. J. Symp. (Int.) Combunt., [ h o c . ] ,22 1988,125-134. (5)(a) Vaatola, F. J.; Pirone, A. J. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1966,10,63.(b) Vaatola, F. J.; McGahan, L. J. Fuel 1987, 66, 886.
0887-0624/91/2605-0436$02.50/0
and even plasma pyrol~sis.~ A preferred technique for generating the fastest heating conditions utilizes the conversion of high-intensity light into thermal energy in an appropriate substrate. Granger and Ladne9 were the first to show that enhanced yields of low molecular weight (MW) gases resulted from ultrarapid flashlamp pyrolysis. Their studies revealed that enhanced yields of gases are obtained with increasing flash intensity for high vitrain coal particles from Coal Rank Code 902 to Coal Rank Code 203 in a flashlamp pyrolyzer. In an independent study, Calkins observed that flash pyrolysis of Pittsburgh No. 8 coal a t >700 "C produces low molecular weight hydrocarbons such as acetylene, ethylene, and propylene? while entrained flow pyrolyses of the same substrate at 445 "C, produced only aromatic compounds (6)(a) Calkins, W.H.; Tyler, R. J. Fuel 1984,63,1119.(b) Calkins, W . H.Energy Fuels 1986,1,59. (7)Bittner, D. K.; Baumann, H.; Klein, J. Fuel l9M,64, 1370. (8)Granger, A. F.; Ladner, W. R. Fuel 1970,49,17. (9)(a) Calkins, W.H. Prepr. Pap.-Am. Chem. SOC.Diu. h l Chem. 1983, 28, 85. (b) Calkins, W.H.; Hoveepian, B. K.; Drykacz, G. R.; Bloomquiet, C. A. A.; Ruecic, L.Fuel 1984,63,1226.
0 1991 American Chemical Society
Ultrarapid Flashlamp Pyrolysis
containing polymethylene chains. Vastola and co-workers have shown that a high-intensity laser (i.e., ruby, C02 lasers) may also be used as a pyrolysis source for coals over a wide range of rank and oil shales: yielding hydrogen, acetylene, and ethylene with some amounts of naphthalene, benzene, and toluene among other liquid/solid products being produced. Hanson's group has observed similar resulta using a pulsed ruby laserloto pyrolyze Texas lignite, New Mexico subbituminous, Louisiana lignite, Northern Illinois No. 6, and Central Illinois No. 6 coals. Model compound studies also show that different products may be obtained when the temperature conditions are varied. For example, acetone at 500 "C yields CHI and ketene, but acetylene (C2H2)and methane are produced at 1000-1500 OC.ll Similarly, acetates containing appropriate substitution undergo an electrocyclic reaction yielding carboxylic acids and alkenes at temperatures of 450-650 OC whereas loss of CO becomes the product at temperatures greater than 925 O C . 1 2 These studies demonstrate that different products are obtained when ultrarapid pyrolysis conditions and traditional pyrolysis techniques are used. In trying to evaluate the potential mechanisms leading to these different products, a critical question concerns whether the chemical reactions originate from photochemical or thermal origins. Surprisingly, this question has never been subjected to experimental test. For flashlamp reactions, photochemical reactions may originate from two photon excitation under high light intensity conditions,13 or may originate from leakage of UV light through the 500-nm cutoff filter. In either event, reactions of photochemical origin would cloud the interpretation of flashlamp pyrolysis designed to emulate ultrarapid heating sources. We report here our studies to determine the origin of the products using flashlamp conditions. Our experimental design was to utilize compounds having known photochemical and thermal reaction products. Bibenzyl (BB) was selected as a model compound because of the large amount of literature data that has been accumulated for both thermal and photochemical excitation. Both thermal and photochemical excitation result in cleavage of the central C-C bond, leading to similar product distributions. Of particular interest are those compounds which give different products when the various excitation sources are used. We have selected 1,Sdiphenylhexane (DPH) because it should be photochemically inert but has been reported to yield products14 when 750 O C pyrolysis conditions are used. Further, we have selected 1,4-dibenzoylbutane (DBB) because this compound has distinguishable photochemical products16 but no products identifiable from 650 OC pyrolysis. Using these three compounds should allow thorough evaluation of the thermal or photochemical origins of the products.
Energy & Fuels, Vol. 5, No. 3, 1991 437
Figure 1. Product distribution from BB as a function of energy density .
radical coupling and disproportionation products depending upon the exact conditions used (eq 1).le1*
-Q.s.6+ aq+y%g 0
BB
B
T
EB
15%
66%
17%
0
minor
(10) Hanson, R. L.; Vanderborgh, N. E.; Brookins, D. G. Anal. Chem. 1977,49,390.
In our laboratory, using slower heating rates, we have verified that pyrolysis of bibenzyl (BB) yields a product distribution similar to that reported earlier by other workers (vide supra) with toluene (T,65%), ethyl benzene (EB, 17%), and benzene (B, 15%) being the major products (eq 1).le1*We note, as have other workers, that the product distribution is highly dependent on the reaction surface in the hot zone. The present product distribution is obtained on the surface of freshly cleaned quartz beads. Pyrolysis using steel wool gave very different results, with much more benzene being formed than reported here. In strong contrast to these low heating rate pyrolysis resulta is the product distribution obtained from ultrarapid pyrolysis using the flashlamp reactor. As seen in eq 2, the major product becomes C2Hz(60%) with minor amounts of CO, T, and B being produced from flashlamp excitation of BB coated onto 23-jtm graphite particles as described in the Experimental Section. A good mass balance is obtained in this reaction, since all of the BB that has reacted is accounted for in the product gases. The amount of reaction varies linearly with pulse energy, with ca. 40% of the BB reacting to form the observed products at the highest pulse energy (i.e., 3.0 J/cm2). The product distribution varies slightly with applied energy density as shown in Figure 1. The change in the identity of the products cannot be simply explained by photochemical
Chem. 1963,1393. (12) Rummens, F. H. A. Recueil 1964,83,901. (13) Scaiano, J. C.; Johnston, L. J.; McGimpwy, W. G.; Weir, D. Acc. Chem. Res. 1988,21,22. (14) Sweeting, J. W.; Wilshire, J. F. K. A u t . J . Chem. 1962, 89. (15) Okumura, Y.; Fuke, K.; Furukawa, S. Rep. Fac. Sci., Shizuoka Univ. 1973,8, 37.
(16) Buchanan 111, A. C.; Dunstan, D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. SOC.1986,108,7703. (17) Poutsma, M. L.; Douglas, E. C.; Leach, J. E. J . Am. Chem. SOC. 1984,106, 1136. (18) (a) Stein, S.; Syryan, M. M. J. Phys. Chem. 1989,93,7362. (b) Stein, S. E.; Robaugh, D. A.; Alfieri, A. D.; Miller, R. E. J. Am. Chem. SOC.1982, 104,6567.
Results Bibenzyl. The thermal and photochemical reaction pathways of bibenzyl are well-known to yield toluene, stilbene, and benzene with relatively small amounts of
(11) (a) Jayaswal, B. K.; Mallikarjuan, M. M.; Hueeain, S. 2.Indian J. Technol. 1984,22, 306. (b) Streith, J.; Tschamber, T. Liebigs Ann.
~~~~~~
~~
Penn et al.
438 Energy & Fuels, Vol. 5, No. 3, 1991
reactions of the BB, since independent solution-phase photolysis of BB results in toluene (83%), benzene (5%), and ethylbenzene (ti%),products originating from the initial formation of the benzyl radical (eq 3). An extremely
+C
H---H
00 I
Ea H
m
+ H>C=C
