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Energy & Fuels 1992,6,172-175
duced, including those which are still trapped in the macromolecular structure of the coal. Furthermore, it has been shown39 that successive chloroformic extractions could increase the extract yield of natural coals by a factor 3 without modifying ita composition. More detailed investigations have revealed that the isomerization rate of hopanoids was higher in natural serie~.~OSuch retarding effects have also been shown by Eglinton et alaa1They have been generally attributed to thermodynamic effects related to the high temperatures or fast heating rates used in laboratory simulation^.'^ On the other hand, it has been shown that the isomerization rate of free hydrocarbons, even trapped in the coal structure, was higher than the isomerization rate of hydrocarbons still attached to the organic macrom~lecule.~~ Then, the hopanoids generated during the first stage of catagenesis and trapped inside the kerogen will undergo a faster isomerization than those still linked to the coal structure. During the main phase of oil production, these trapped hydrocarbons will be mixed with hydrocarbons generated from the kerogen and the mixture will display an higher isomerization degree than expected. Then, the trapping of hydrocarbons into the macromolecular structure of naturally matured coals induces a delay between their formation and their expulsion43and can be responsible for the enhancement of several chemical reactions which will increase their maturity compared to that of the insoluble residue. Such mechanisms can be inferred in order to explain the differences in the aromatization, desubstitution and condensation rates of aromatic hydrocarbons observed during (39) Monthioux, M. Maturation naturelle et artificielle d'une drie de charbons homoghes. Ph.D., Orleans University, 1986. (40) Monthioux, M.; Landais, P. P. Chem. Geol. 1989, 75, 209-226. (41) Eglinton, T.I.; Rowland, S.J.; Curtis, C. D.; Douglas, A. G. Org. Geochem. 1986,10, 1041-1052. (42) Rowland, S.J.; Aareskjold, K.; Xuemin, G.; Douglas, A. G. Org. Geochem. 1985,10, 1033-1040.
the catagenetic phase. As a matter of fact, naturally and artificially p r o d u d aromatics behave similarly during the diagenetic phase (up to 81% COC) whereas, during catagenesis, similar mechanisms are noticed but are retarded in the artificial maturation series. This specific problem has not been encountered when pyrolyzing type 11kerogens which do not display the so-called two-phases structure of type I11 ~0als.4~
Conclusions Such multidisciplinary spectroscopic analysis facilitates the acquisition of structural data on extractable fractions of coals in order to get a better understanding of the mechanisms of hydrocarbons formation. The crosschecking of the data deduced from the different techniques allows the evolution of the structural parameters to be more accurately interpreted. Especially, the interpretation of the UV SFS spectra can be considerably improved by the knowledge of the ring substitution pattern and of the length of the substituents.% It also provides additional data which allow discrepancies between natural and artificial maturation to be evidenced. Most of these differences are related to the peculiar structure of coals which creates a cage effect and induces the trapping of hydrocarbons. Higher temperatures used in confined pyrolysis experiments provoke the slackening of the coal structure and an easier expulsion of the generated hydrocarbons. Such a process can be responsible for the different rates of aromatization and condensation noticed for natural and artificial maturation series. Acknowledgment. This work has been supported by the COPREP (No. G/10011/90) and by the INSU (No. 91 ATP 645). We gratefully acknowledge C. Moreaux and P. Doumenq for their technical and scientific assistance. (43) Behar, F.;Vanderbroucke,M. Org. Geochem. 1987,13,927-938.
N20Emissions from a Fluidized Bed Catalytic Cracker D.A. Cooper* and A. Emanuelsson Swedish Environmental Research Institute, Box 47086, S-402 58, Gothenburg, Sweden Received September 5, 1991. Revised Manuscript Received December 27, 1991
Measurements of primarily N20 have been carried out at a fluidized bed catalytic cracker employing a zeolite catalyst to degrade desulfurized vacuum-distilled gas oil. In view of the relatively low combustion temperatures (ca. 700 "C) and high NO, levels (340-520 ppm) in the catalyst regenerator section, the N 2 0 levels were surprisingly low, 3-25 ppm. In addition, only trace amounts of hydrocarbons and sulfur-containing species were measured.
Introduction The average atmospheric nitrous oxide concentrations (N20)concentrations are presently at ca. 300 ppb and have been increasing at a rate of 0.1&0,3% per year.12 Since (1) Rasmussen, R. A,; Khalil, M. A. K. Tellus 1983, 35B, 161-169.
0887-0624/92/2506-0172$03.00/0
N20 has an important role in causing global climatic changes, significant worldwide concern has arisen. In the trOPosPhere, N20, being a relatively strong absorber of infrared radiation and relatively long-lived (ca. 150 year (2) Weiss, R. F.J . Geophys. Res. 1981,86,7185-7195.
0 1992 American Chemical Society
N20from a Fluidized Bed Catalytic Cracker life time), can contribute to global warming through the so-called “greenhouse effect”. Present estimates show that among the other greenhouse gases (C02, CHI etc.), N20 is responsible for ca. 6% of the total warming effect.3 Due to the stability of N20 in the troposphere, transport to the upper atmosphere or stratosphere is possible. In this situation, both the photolytic decomposition of N20 and ita reaction with 0 radicals occur whereby nitric oxide, NO, is formed which subsequently reacts in a catalytic cycle removing ozone, In the stratosphere this depletion of the protective ozone layer has caused considerable alarm since it leads to an increase in the harmful UV radiation reaching the earth’s surface. Estimates of the anthropogenic sources of N20 have roughly been given as one-third of the total global N20 production and these sources are suspected to be responsible for the rising atmospheric concentrations. The use of fertilizers and the combustion of fossil fuels constitute the man-made sources but their relative significance in connection with the increasing atmospheric concentrations has not entirely been r e ~ o l v e d . ~ As . ~ far as stationary combustion of fossil fuel is concerned, the only significant source to date is from fluidized bed boilers used for power and heat generation. In this application, which is normally renowned for low NO, formation, the inherent low combustion temperatures (800-900 “C)and the fuel-nitrogen content of ca. 0.5-1.5% favor much higher N20 emissions in comparison to other stationary combustion sources. Similar combustion environments to that in fluidized bed boilers can also exist in other industrial sectors, which as yet have not been investigated as potential N20 sources. An example is the fluidized bed catalytic cracking process used within the petroleum refinery industry.6 Oil Refining and the Role of Fluidized Bed Catalytic Crackers. Modern oil refining employs several processes whereby crude oil is converted to marketable products. The initial step common to almost all refining operations is a primary atmospheric distillation where the lighter fractions separated are of most commercial value. The fate of the heavier fractions, however, is one or several basic degradation alternatives (coking, vacuum distillation, fluidized catalytic cracking, hydrocracking, etc.), depending on economic factors and the quality of the crude oil feedstock. These pathways can be extended further by additional processes designed to improve the quality of the feedstock, e.g., visbreaker, desulfurization units, etc. At present, fluidized catalytic cracking represents in many cases the most profitable option as a result of increasing crude oil prices and increasing demands for automobile It is estimated that the present capacity of catalytic crackers in USA and western Europe is ca. 8 million barrels per day.g Within the fluidized catalytic cracking process, zeolitebased catalysta are partially used to degrade or “crack” the heavier fractions of the oil to lighter and more useful hy(3) Houghton, J. T., Jenkins, G. J., Ephraums, J. J., Eds. Climate Change: The ZPCC Scientific Assessment; Cambridge University Preas: Cambridge, England, 1990. (4) Proffitt, M. H.; Fahey, D. W.; Kelly, K. K.; Tuck, A. F. Nature 1989,342, 233-237. (5) European Workshop on N20 Emissions, LNETI, June 1990, Lishnn.
(6) Lyon, R. C.; Kramlich, J. C.; Cole, J. A. Enuiron. Sci. Technol. 1989,23; 392-393. (7) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes;McGraw-HillChemical Engineering Series; McGraw-Hill: New York, 1979; pp 1-107. (8) Bhatia, S. Zeolite Catalysis: Principles and Applications;CRC Press: Boca Raton, FL, 1990; pp 241-250. (9) Ottersted, J.-E.;Gevert, B.; Sterte,J. Kem. Tidskr. 1991, I , 31-38 (in Swedish).
Energy & Fuels, Vol. 6, No. 2, 1992 173 Table I. Typical Operating Conditions for the Reactor and Regenerator Sections of a Fluidized Catalytic Crackeld reactor base temp, OC top temp, OC press., atm catalyst to oil ratio gas residence time, s regenerator temp in cyclone, OC CO/C02mole ratio press. at bottom of fluidized bed, atm superficial gas velocity, cm/s solids residence time, s coke content of catalyst at entrance, w t % coke content of catalyst at exit, w t %
550 510 3 61 5-7 650-760 0.7:l-1.3:l 3.5 60 30 0.8 0.1
Table 11. Analysis of the Cracking Catalyst 0.86 0.30 145
