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Energy & Fuels 1992,6, 264-271
of 8- and y-protons to mostly a-protons. (3) The major CI-C3 hydrocarbons are methane, ethylene, and acetylene, but oils are the major hydrocarbon gaseous product from bituminous coals. Yields of all gaseous hydrocarbons, carbon oxides, and water increase in proportion to the tar yields, with no distinctive surges or lapses. (4)Aromatic hydrocarbons, especially tars, are the only shuttles for fuel nitrogen. During the later stage of primary devolatilization (1)No tars are expelled but additional C l X 2compounds are generated, and distinct surges in the evolution of CO and HzO from coals of lower rank actually increase rates of weight loss. H/C ratios for char from 0.3 and 0.4, and volatilization of at least three-fourths of the coal oxygen
signal the end of primary devolatilization. (2) HCN appears for the first time. Acknowledgment. This research was sponsored by the Electric Power Research Institute (EPRI), under their Exploratory Research Program. We are also grateful to the Link Foundation for providing an Energy Fellowship to J.C. during academic year 1988-89. Technical contributions from C. Castagnoli for the 'H NMR analyses, from S. Cho for the chromatographic methods, and from C.-W. Lau for the calculated thermal histories are gratefully acknowledged. Registry No. CHI, 74-82-8; HzC=CHz, 74-85-1; H C S H , 74-86-2; CO,, 12795-06-1; HzO, 7732-18-5; CHSCHS, 74-84-0; H(CHZ),H, 74-98-6; H&=CHCHS, 115-07-1.
Coal Devolatilization during Rapid Transient Heating. 2. Secondary Pyrolysis John C. Chen, Craig Castagnoli, and Stephen Niksa* High Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University, Stanford, California 94305 Received October 7, 1991. Revised Manuscript Received January 16, 1992
This study utilizes the radiant coal flow reactor described in part 1 in an operating mode which independently regulates the extent of secondary pyrolysis. Product distributions for transient secondary pyrolysis at nominal heating rates of lo4 K/s are reported for subbituminous and hvA bituminous coals, to characterize the transformations involving oxygen, nitrogen, and aliphatic functional groups as coal volatiles are pyrolyzed into soot. They show that the sum of the yields of tar,oils, and soot remains invariant at the ultimate yields of tar plus oils for primary devolatilization. Ultimate soot yields can reach one-third of the coal mass for hvA bituminous coals. The aggregate amounts of C1-C3 hydrocarbons change very little while all species are eliminated except acetylene. Among oxygenated gases, CO yields increase steadily and then surge while H20 and COz yields pass through weak maxima. Tar aromaticities increase dramatically, reaching ultimate values greater than two-thirds, and tar H/C ratios decrease, reaching 0.75. Soot is continuoilsly graphitized, and ita C/H ratio covers the same range as in flames, from 2 to 10. Up to one-fourth of the coal nitrogen expelled with volatiles during primary devolatilization is incorporated into soot during secondary pyrolysis for three different coals. All nitrogen is incorporated very early, and the total amount of coal nitrogen in soot remains constant even while soot yields increase dramatically. Nitrogen incorporation into soot traces the importance of direct conversion of tar into soot during the initial stages. Thereafter, substantial soot mass is added from C1-C3 hydrocarbons, probably acetylene. Since heteroatoms are expelled from tar and the tar/oils plus soot mass is invariant, the light addition species cannot come entirely from a tar decomposition product.
Introduction In any practical application, the primary products of coal devolatilization are radically transformed by secondary chemistry after they escape into the gas phase. Tars in hot gases have a propensity for sooting, which has important practical consequences. Soot forms luminous mantles around burning coal particles that emit radiation very effectively. Sooting also releases substantial amounts of hydrogen from soot precursors and aliphatic hydrocarbons, which accelerates burnine: velocities during combustion'and raises the heating valLe of gasification-products. Secondary pyrolysis also expels and transforms 0887-0624/92/2506-0264$03.00/0
noxious heteroatomic compounds, and creates organic mutagens. Previous experimental work at moderate temperatures established the sequence of major transformations. In the most common scheme, fixed or fluidized beds generate primary volatiles for more severe thermal processing in separate flow reactors. Provided that the primary pyrolyzer is cooler than 825-875 K, its products appear to be relatively free of secondary transformation^.'-^ Results (1) Serio, M. A.; Peters, W. A,; Howard, J. B.Ind. Eng. Chem. Res. 1987,26,1831.
0 1992 American Chemical Society
Coal Devolatilization
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show that tars decompose into oils and noncondensibles below loo0 K and then form carbonaceoussolids at higher temperature^.'-^ The distribution of noncondensible hydrocarbons becomes progressively less complex until only acetylene remains.lg Above loo0 K oxygenated aromatics expel C02and nitrogen-containingaromatics expel HCN,45 paralleling the surges in CO and HCN production during the final stages of primary coal devolatilization seen in earlier work6,' and in part 1of this study. None of these changes has yet been resolved in time for reaction rate determinations, but their characteristic times are surprisingly long, ranging from 1 to 5 s at lo00 K. Secondary pyrolysis occurs on millisecond time scales under simulated pulverized fuel (pf) firing conditions in entrained-flow reactors,g10 yet follows the same basic sequence of events. In entrained-flow reactors, the total amount of tar plus soot remains constant throughout secondary pyrolysis, consistent with the view that tar is the precursor to soot.QJOEntrained-flow studies have also shown that complex mixtures of polycyclic compounds are converted into a handful of unsubstituted 3-6-ring compounds.'O This study characterizes the transformations involving oxygen, nitrogen, and aliphatic peripheral groups in coal volatiles being pyrolyzed into soot during rapid transient heating. It utilizes the radiant furnace introduced in part 1 in an operating mode that independently regulates the extent of secondary pyrolysis over nearly ita entire range. Being free of secondary transformations, the distributions of primary devolatilization products in part 1provide an essential reference for the results in this paper. Product distributions for secondary pyrolysis at nominal heating rates of lo4 K/s are reported for subbituminous and hvA bituminous coals, with selected results for a low-volatility coal. Major noncondensibles are resolved into C1-C4 hydrocarbons, oils, CO, C02, and H20. Fuel nitrogen evolution is monitored with HCN and NO yields and the nitrogen contents of char, tar and soot. Tar quality is gauged by ita elemental C/H/N distribution and proton distributions from NMR spectroscopy. Soot yields are based on insolubility in tetrahydrofuran (THF). Yields and elemental compositions of soot and char close the balances on total mass,carbon, and nitrogen in individual runs.
Experimental Section The radiant coal flow reactor is described in part 1, so this section explains only the new operating mode and its thermal histories, and analyses involving soot. Two operating conditions are adjusted to promote secondarychemistry: (1) the coal loading at the inlet is quadrupled to -1200 particles/cm3,which imparts similar heating rates and ultimate temperatures to the suspension and its entrainment stream because higher loadings increase the interfacialarea for their convective heat transfer; and (2) the length of the hot zone in the furnace is increased by 21/2 times to 12.5 cm, to extend residence times to 175 ma. In this operating mode, the severity of the thermal processing is regulated simply by (2) Nelson, P. F.; Tyler, R. J. Symp. (Int.)Combust. [Proc.],21 1986, 427. (3) Xu, W.-C.; Tomita, A. Fuel Process. Technol. 1989, 21, 25. (4) Axworthy, A. E.; Dayan, V. H.; Martin, G. B. Fuel 1978,57, 29. (5) Bruimma, 0. S.L.; Geertama, R. S.; Bank,P.; Moulijn, J. A. Fuel 1988, 67, 334. (6) Freihaut, J. D.; Zebielski, M. F.; Seery, D. J. Symp. (Int.) Combust. [Proc.],19 1982, 1159. (7) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 632. (8) Ubhayakar, S. K.; Stickler, D. B.; von Roeenberg, C. W., Jr.; Gannon, R. E. Symp. (Int.) Combust. [Proc.],16 1977, 427. (9) Nenniger, R. D. Aerosols Produced from Coal Pyrolysis; Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, 1986. (10) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Symp. (Int.)Combust. [Proc.],22 1988, 135.
Energy &Fuels, Vol. 6, No. 3, 1992 265 1400
Y
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.-7
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Figure 1. Calculated thermal histories of particles (solid curves) and gas (dotted curves) for secondary pyrolysis with 1200 particles/cm3 entrained at 18 cm/s at the inlet. The letters denote furnace temperatures and measured residence times as follows: (a) 1480 K, 174 ms; (b) 1570 K, 170 ms; (c) 1660 K, 167 ms; and (d) 1740 K, 156 ms. increasing the furnace temperature in successive runs with the same inlet gas velocity and suspension loading. The thermal histories among such cases have similar heating rates but different reaction times and ultimate temperatures. Temperature profiles for the experiments reported in this paper appear in Figure 1. Under these conditions the suspension initially heats faster than the gas, but the high convective heat transfer rate quickly brings the particle and gas heating rate into balance at nominal values of lo' K/s. As a result, the temperature difference is lees than 200 K at the outlet of the 1740 K furnace. Ultimate suspension temperatures are usually much hotter in this operating mode than in the primary devolatilizationstudies (cf. Figure 2b, part 1). Nevertheless, the most severe condition for primary devolatilizationof 89 ma in an 1840 K furnace imparta an outlet temperature of 950 K, which is close to the value of 980 K for the least severe secondary pyrolysis condition of 175 ms through the furnace at 1480 K, in Figure 1. At the most severe condition the maximum particle temperature is 1350 K. Gas temperatures are elevated even more in this operating mode. Calculated gas temperatures were always cooler than 600 K in the primary devolatilization studies but reach 1170 K in the longer furnace at 1740 K. Measured residence times for furnace temperatures from 1480 to 1740 K range from 175 to 155 ms, decreasing with increasing wall temperature due to the thermal expansion of the entrainment stream. Except for the above changes in the operating conditions, the hardware and procedures described in part 1 are implemented here. Soot is recovered from the aerosol by extraction and filtration. Aerosols on the glass-fiber filters in the virtual impactor are first extracted into tetrahydrofuran (THF) in an ultrasonic bath; then the solution is passed through a Teflon filter with 0.2-pm pores. The dry weight of the membrane residue is denoted as the soot yield. The solutions are stored in amber glass vi& under nitrogen until they are processed to recover pure tar samples. They are concentrated in a Kuderna-Danish concentrator to 0.3 mL before the remaining solvent is evaporated with the procedure of Lafleur et al." Details of the extraction and concentration procedures are provided elsewhere.12 Oxygen balances will be reported for products from the subbituminous coal. Oxygen contents of char, tar,and soot were inferred from the weights of residues in the C/H/N elemental determinations corrected for ash in the char with the level in the whole coal, assuming negligible ash vaporization. A similar correction for sulfur is inconsequential because there is only 0.4 wt % sulfur in the whole coal, and therefore virtually none in char, tar, or soot. (11) Lafleur, A. L.; Monchamp, P. A.; Plummer, E. F.; Kruzel, E. L. Anal. Lett. 1986, 19(21, 22), 2103. (12) Chen, J. C. Effects of Secondary Reactions on Product Distri-
bution and Nitrogen Evolution from Rapid Coal Pyrolysis Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Stanford, 1991.
Chen et al.
266 Energy & Fuels, Vol. 6,No. 3, 1992
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S 8 ^^ Aside from these mechanistic implications, incorporation of nitrogen into soot is a heretofore unrecognized aspect of nitrogen evolution during the initial stages of pulverized coal combustion. It has the potential to substantially reduce the amount of coal nitrogen amenable to aerodynamic NO, abatement strategies for coals with large tar yields, as described in a separate paper.lg
Conclusions Distributions of the major products of secondary pyrolysis of subbituminous and hvA bituminous coals during rapid transient heating show the following: (1) The sum of the yields of tar, oils, and soot remains invariant. Ultimate soot yields equal the ultimate yields of tar plus oils for primary devolatilization and can reach one-third of the coal mass for hvA bituminous coals. (2) The aggregate amounts of C1-C3 hydrocarbons change very little while all species are eliminated except acetylene. Among oxygenated gases, CO yields increase steadily and then surge, while H 2 0 and C02 yields are perturbed slightly by water gas shift equilibrium. (3) Tar aromaticities increase dramatically, reaching ultimate values greater than two-thirds, and tar H/C ratios (19)Chen, J. C.; Niksa, S. Symp. (Znt.)Combut. [Proc.],24, in preae.
(20) Houser, T. J.; McCarville, M. E.; Biftu, T. Znt. J. Chem. Kinet. 1980, 12, 555.
Energy & Fuels 1992,6,271-277 decrease, reaching 0.75. Proton distributions shift as 8and y-protons are eliminated and a-protons become less abundant. Soot is continuously graphitized, so that its C/H ratio covers the same range as in flames, from 2 to 10. Consequently,H2eventually becomes a major product of secondary pyrolysis. (4) Up to one-fourth of the coal nitrogen expelled in volatile5 during primary devolatilization is incorporated into soot during secondary pyrolysis for coals whose volatiles are dominated by tar. Ten percent is incorporated for the subbituminous coal. All nitrogen is incorporated early, and the totalamount of coal nitrogen in soot remains constant even while soot yields increase dramatically. (5) Nitrogen incorporation into soot traces the importance of direct conversion of tar into soot during the initial
27 1
stages. Thereafter, substantial soot mass is added by C1-C3 hydrocarbons, probably acetylene. The light addition species cannot come entirely from a tar decomposition product.
Acknowledgment. This research was sponsored by the Electric Power Research Institute (EPRI), under their Exploratory Research Program. We are also grateful to the Link Foundation for providing an Energy Fellowship to J.C. during academic year 1988-89. Technical contributions from S. Cho for the chromatographicmethods and from C.-W. Lau for the calculated thermal histories are gratefully acknowledged. Registry No. H2, 1333-74-0;CH=CH, 74-86-2.
Pyrolysis of Wood Specimens with and without Minerals: Implications for Lignin Diagenesis? K. Ohtax and M. I. Venkatesan* Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California NO24 - 1567 Received October 7, 1991. Revised Manuscript Received February 28, 1992
In order to evaluate lignins as geochemical tracers of paleovegetation, thermal alterations of lignins and the yields of their phenolic residues were investigated by laboratory pyrolysis experiments. Gymnosperm and angiosperm woods as well as humic acids from peata were pyrolyzed at 200 OC with and without clay and other minerals. The pyrolyzed samples were oxidized with alkaline CuO to release phenolic compounds which were derivatized to their silyl ethers and quantitated by gas chromatography (GC) and GC/mass spectrometry. The yields of lignin phenolic residues after thermal degradation folbwed roughly first-order kinetics. Except for the accelerated degradation by acidic montmorillonite, the kinetic plots with other minerals examined were almost superimposable on the plots for the pyrolysis of wood without minerals suggesting their inability to catalyze lignin degradation. Simple aromatic C-0 bond cleavage resulting in demethoxylation of lignin phenolic residues seems unlikely. The first step of the multistep reactions, involving 0-C bond cleavage leading to demethylation, is probably a dominant process, after which dehydroxylation results in demethoxylation. The ratio of syringyl to vanillyl (S/V) phenolic residues was thermally modified. However, the ratio remains constant at later stages of pyrolysis, indicating that the S/V ratio can, to a certain extent, be used as a qualitative index of paleovegetation and paleoclimatic indicator as well. The bonding at a-carbon in side chain is thermally unstable, showing an increase in phenolic acid to aldehyde ratio with pyrolysis time. This implies that acid/aldehyde (ACD/ALD) ratios in geological specimen present an integrated signal of microbial as well as thermal effects. The data obtained in the present study help define the changes in the lignin compositional trends that characterize the early stages of coalification. Introduction Lignins are polymers biosynthesized only by vascular plants from substituted cinnamyl alcohol units with phydroxyphenyl,vanillyl, or syringyl residues. Proportions of these alcohols differ between gymnosperm and angiosperm lignins. Gymnosperm lignins are made up almost solely from coniferyl alcohol (vanillyl) and angiosperm lignins from both coniferyl and sinapyl alcohol (syringyl),
* Author to whom correspondenceshould be addressed. 'Institute of Geophysicsand Planetary Physics Contribution No.
3568. Present address: Water Research Institute, Nagoya University, Furc-cho, Chikusa-ku, Nagoya 464, Japan. f
0887-0624/92/2506-0271$03.00/0
and grass lignins have high proportions of the coumaryl alcohol (p-hydroxyphenyl) units.'P2 Hence substitution patterns of simple phenols produced by mild oxidation of lignins have been used as a geochemical tracer in identifying vegetation types which contribute to the land-derived organic material in coastal to reconstructing paleo~egetations,"~ and in characterizing fossil planta."'O (1) Grisebach, H. Naturwissenschaften 1977, 64, 619-625. (2) Hedges, J. I.; Mann, D. C. Geochim. Cosmochim. Acta 1979,43,
1803-1807. ( 3 ) Hedges, J. I.; Mann, D. C. Geochim. Cosmochim. Acta 1979,43, 1809-1818. (4) Hedges, J. I.; Clark, W. A.; Cowie, G. L. Limnol. Oceanogr. 1988, 33,1116-1136.
0 1992 American Chemical Society
