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 chromatographic methods 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 correspondence should be addressed. 'Institute of Geophysics and 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
272 Energy & Fuels, Vol. 6, No. 3, 1992
Lignins are generally minor components in tree leaves and needles and are easily decomposed during early diagenesis in aquatic en~ironments."-'~ In woods, lignins occur as a major component representing 20-3590 of cell walls.' Wood lignins are microbially degraded in sedimentary environments more slowly relative to carbohydrates or nitrogen-containing compound^.'^-^^ Relative abundances of p-hydroxyphenyl, vanillyl, and syringyl residues in original wood lignins are modified to some extent by microbial alterations including depolymerization, demethoxylation, demethylation, and further defunctionalization of the phenolic residue^."^^^ Similar alterations of lignin remains are believed to occur in subsequent geochemical processes during diagenesis and catagenesi~.~*~~ Geochemical alterations, in addition to microbial ones, of lignin phenols may lead to marked changes in the information about the original input of plant material from land vegetation.'O Mrazikova et al.25 noticed that methoxy groups in coals are thermally labile. The pyrolysis experiments conducted by Hayatsu et al.26 indicated that demethoxylation of gymnosperm lignins could occur rapidly. However, the earlier studies provided no information about the alteration of syringyl residue which is an ultimate indicator of angiosperm lignins. Further, detailed studies of geochemical alterations of lignin structures, especially of phenolic residues and their diagenetic stability, are required for evaluating lignins as a geochemical tracer of paleovegetation types and as a potential paleoclimatic indicator as well. The current project attempted to address some of these aspects. This study examines lignin degradation products after short-term pyrolysis of both gymnosperm and angiosperm plants as well as humic acids isolated from peats to follow the geochemical changes of lignin components during (5) Hedges, J. I.; Ertel, J. R.; Leopold, E. B. Geochim. Cosmochim. Acta 1982,46, 1869-1877. (6) Leopold, E.; N i c k " , R.; Hedges, J. I.;Ertel, J. R. Science 1982, 218, 1305-1307. (7) Ishiwatari, R.; Uzaki, M. Geochim. Cosmochim. Acta 1987, 51, 321-328. (8) Leo, R. F.; Barghoorn, E. S. Science 1970, 168, 582-584. (9) Sigleo, A. C. Science 1978,200, 1054-1056. (10) Mycke, B.; Michaelis, W. Naturwksenschaften 1986,73,731-734. (11) Hedges, J. I.; Weliky, K. Geochim. Cosmochim. Acta 1989, 53, 2659-2673. (12) Benner, R.; Weliky, K.; Hedges, J. I. Geochim. Cosmochim. Acta 1990.54. 1991-2001. (13) Benner, R.; Hatcher, P. G.; Hedges, J. I. Geochim. Cosmochim. Acta 1990,54, 2003-2013. (14) Merer, P. A.; Leenheer, M. J.; Erstfield, K. M.; Bourbonniere, R. A. Nature-1980,287, 534-536. (15) Hatcher, P. G.; Breger, I. A.; Earl, W. L. Org. Geochem. 1981,3, 49-55. (16) Hatcher, P. G.; Spiker, E. C.; Szeverenyi, N. M.; Maciel, G. E. Nature 1983,305, 498-501. (17) Hedges, J. I.; Cowie, G. L.; Ertel, J. R.; Barbour, B. J.; Hatcher, P. G. Geochim. Cosmochim. Acta 1985,49, 701-711. (18) Benner, R.; Fogel, M. L.; Sprague, E. K.; Hodson, R. E. Nature 1987, 329, 708-710. (19) Spiker, E. C.; Hatcher, P. G. Geochim. Cosmochim. Acta 1987, 51, 1385-1391. (20) Stout, S. A.; Boon, J. J.; Spackman, W. Geochim. Cosmochim. Acta 1988,52,405-414. (21) Hayatsu, R.; Scott, R. G.; Winans, R. E.; Moore, L. P.; McBeth, R. L.; Studier, M. H. Nature 1979,278,41-43. (22) Hayatsu, R.; Winans, R. E.; McBeth, R. L.; Scott, R. G.; Moore, L. P.; Studier, M. H. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Adv. Chem. Ser.; American Chemical Society: Washington, DC, 1981; Vol. 192, pp 133-149. (23) Niklas, K. J.; Pratt, L. M. Science 1980, 209, 396-397. (24) Hatcher, P. G.; Breger, I. A.; Szeverenyi, N.; Maciel, G. A. Org. Geochem. 1982, 4, 9-18. (25) Mrazikova, J.; Sindler, S.; Veverka, L.; Macak, J. Fuel 1986, 65, 342-345. (26) Hayatsu, R.; McBeth, R. L.; Scott, R. G.; Botto, R. E.; Winans, R. E. Org. Geochem. 1984,6,464-471.
Ohta and Venkatesan
laboratory thermal alteration. The wood specimens were also intimately mixed with clay and other minerals (Le., montmorillonite, pyrite, etc.) and pyrolyzed since clay minerals, especially, are known to play an important role in thermal alteration of organic c ~ m p o ~ n d To s . check ~~~~ the possibility of stepwise demethoxylation of lignin phenolic residues which is pertinent to the information of plant source, a lignin polymer with aromatic residues with minimal p-OH groups is required. Fresh plant lignins are preferred from this point of view to biologically degraded material which invariably contains significantly larger amounta of p-OH residues. Further, it is almost impossible to single out lignin polymer that is naturally and microbially well-degraded yet geochemically unaltered, because geochemical processes tend to overlap the later stages of biological degradation. This rationale prompted us to choose fresh plant material for thermal alteration instead of microbially degraded material. Since the principal factor for diagenesis of lignin after microbial degradation (asalso discussed previously) is thermal energy,1° pyrolyses of wood specimen were carried out to follow the diagenetic fate of the lignin polymer. Experimental Procedures Samples. Fresh wood chunks of pine tree (Pinuspinea) and African red alder (Cunonia capensis) were drilled to produce wood
dust. The wood dust was freeze-dried and ground with a mortar and pestle to pass a 60-mesh sieve. The fine wood powder was extracted with methanol and subsequently with dichloromethane (CH2Cl&to remove lipid material, and freeze-dried. Dried bases of petiole of pigmy date palm (Phoenix roebelenii) were ground, solvent-extracted,and freeze-dried in a similar manner. The humic acid sample was prepared by the method of Schnitzer and Khanmfrom peats collected at Killamey, Kerry County, Ireland. Pyrolysis. Finely powdered clay minerals (kaolinite,montmorillonite, and acidified montmorillonite (MontmorilloniteK-10 from Aldrich)),pyrite and marcasite were extracted with a mixture of methanol and CH2C12(l:l, v/v). After drying at room temperature, the minerals were heated at 100 O C overnight. Each mineral and a wood sample were mixed at a weight ratio of 41 and ground together. In a 20 X 0.8 cm i.d. Pyrex tube sealed at one end, was placed 20 mg of the wood powder or 100 mg of the wood-mineralmixture. After evacuation,the tube was sealed under vacuum. The tubes (prepared in duplicates) were placed in an oven and heated at 200 "C &0.2 O C ) . Few wood specimenswere also pyrolyzed in quartz tubes along with Pyrex tubes. The lignin phenols' composition in the samples heated for a given time interval (Le., 50 and 150 h) was almost the same from both the Pyrex and quartz tubes and the difference was well within experimental error. This showed that Pyrex tubes did not have significant active sites to muse measurable atrifacta in the chemistry of the pyrolysate. All subsequentpyrolysis experiments were, therefore, conducted in Pyrex tubes which were also easier to handle than quartz. The samples (fresh as well as pyrolyzed) were oxidized with alkaline CuO, and the resultant lignin-derived phenols were separated by the method similar to that described by Steinberg et modified from Hedges and E ~ t e l .Briefly, ~ ~ a mixture of the sample (20-100 mg), CuO (1g), Fe(NH4)4(S04)2.6H20 (100 mg) and 2N NaOH (8 mL) in a stainless-steel vessel (custommade) fitted with a Teflon O-ring and a stainless-steelscrew cap were bubbled with N2stream under ultrasonication, sealed, and heated at 170 "C for 3 h. After cooling, the reaction mixture was spiked with an internal standard (ethylvanillin),and then filtered (27) Hussler, G.; Albrecht, P. Nature 1983, 304, 262-263. (28) Tannenbaum, E.; Ruth, E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1986,50, 805-812. (29) Schnitzer, M.; Khan, S. Humic Substances in the Enuironment; Marcel-Dekker Inc.: New York, 1972. (30) Steinberg, S.; Venkatesan, M. I.; Kaplan, I. R. J. Chromatogr. 1984,298,427-434. (31) Hedges, J. I.; Ertel, J. R. Anal. Chem. 1982, 54, 174-178.
Energy & Fuels, Vol. 6, No. 3, 1992 213
Pyrolysis of Wood Specimens Table I. Composition of Lignin Phenols in Lake Washington Mud H VI1 concentrationa comDd this workb Hedges data' 0.064 BAd 0.108 f 0.039 0.161 p-ALD 0.017 0.036 f 0.023 p-KET 0.373 f 0.084 0.495 V-ALD 0.030 m-HBA 0.119 0.028 0.117 V-KET 0.121 0.115 f 0.024 p-ACD 0.169 f 0.045 0.244 S-ALD 0.060 f 0.015 0.026 S-KET 0.150 f 0.034 0.189 V-ACD 0.049 f 0.014 0.074 S-ACD 0.060 f 0.009 0.102 C 0.055 0.059 f 0.013 F
0
h
-1
0
2
H,CO '@OCH2
\\
-
- 2I 1 -2
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V-TOT
OH
*
aConcentration is in mg/100 mg of OC. bAverage of duplicate analyses. Kindly provided by J. I. Hedges along with the sample. Abbreviated names represent benzoic acid, p-hydroxybenzaldehyde, p-hydroxyacetophenone, vanillin, m-hydroxybenzoic acid, acetovanillone, p hydroxybenzoic acid, syringaldehyde, acetosyringone, vanillic acid, syringic acid, p-coumaric, and ferulic acid, respectively.
-
Time (hr)
Figure 1. Cunonia capensis heated at 200 "C: First-order kinetics plots of syringyl (S-TOT) and vanillyl (V-TOT) phenols. Line corresponding to TOT = S-TOT + V-TOT. R = -CHO, -COCH3, or -COOH.
.-0
I
>
1
through sintered glass disk filter assembly. The filtrate was extracted with CHpC12,then acidified, and extracted with diethyl ether. The diethyl ether extract was concentrated to a small volume (1 mL), 200-pL portions of which was used for trimethylsilyl ether (TMS)derivatizationwith pyridine (30 pL) and BSFTA (30 pL, containing 1%of TMCS) a t 60 "C for 10 min. The TMS derivative (silyl ethers) was analyzed by a Hewlett-Packard 5840 GC equipped with a 60 m by 0.25 mm i.d. fused silica capillary column (DB-1, J&W Co.) and FID. GC oven temperature was programmed from 100 to 290 "C a t a rate of 3 "C/min. Hydrogen was used as a carrier gas. Triplicate analyses of C. capensis showed that the overall procedure has a precision of &3-7% of the measured values for individual lignin phenols. All the phenols were identified by comparing their GC retention behavior and mass spectral data with those of commercial standards. The mass spectra were measured by a Finnigan 4000 quadrupole mass spectrometer interfaced with Finnigan 9610 gas chromatograph using He as a carrier gas. All solvents were glass-distilled and the chemicals used were of reagent grade purity. Distilled ether was stored over ferrous ammonium sulfate. Pyridine was distilled under nitrogen. Standards were purchased from Aldrich and BSTFA from Pierce. Duplicate analyses of Lake Washington mud (H VII) were also carried out. The composition of ligninphenols compared favorably with the data provided by Dr. J. I. Hedges as shown in Table I.
vanillyl phenols ratio after pyrolysis. (b) Acid/aldehyde ratio after pyrolysis.
Results and Discussion Alkaline CuO oxidation of the angiosperm wood C. capensis produced a series of syringyl phenols (syringaldehyde, acetosyringone, and syringic acid) and vanillyl phenols (vanillin, acetovanillone, and vanillic acid). Yields of total syringyl (S-TOT) and vanillyl (V-TOT) phenols corresponded to respectively 8.8 and 2.0% (dry weight basis) of the wood. p-Hydroxyphenyl compounds were detected only in trace amount. The lignin phenol yields decreased markedly when C.capensis was heated alone at 200 "C. Concentrations of both syringyl and vanillyl phenols dropped sharply up to 60 h of heating period (fmt stage) and then gradually thereafter (second stage). No p-hydroxyphenyl compounds were detected in any of the heated samples. Figure 1illustrates the plots of In (Ct/Co), where C, is the yield after time t and Cothe initial yield of lignin phenols against heating time t. I t is clear that the yields of the lignin phenolic residues released after thermal degradation can be expressed by the first-order reactions having at least two different rate constants; a greater rate deduced from the steep slope of the line in the early stage, and a smaller one suggested from the gently
sloping line in the second stage. The plots also indicate that vanillyl residues decrease more quickly than syringyl residues during early hours of heating in the first stage. As a result, the yield of syringyl to vanillyl phenols (S/V ratio) increases from 4.3 to 7.1 within 40 h of heating (Figure 2a) which gradually reaches a plateau and tends to remain constant after 150 h. Many elementary processes such as demethoxylation, demethylation, dehydroxylation, or alkyl substitution could be responsible for thermal degradation of lignin polym e r . * J O ~ ~Among ~ - ~ ~ them, demethoxylation is the most crucial reaction from our point of view because it can directly modify the substitution pattern of phenolic residues in original lignins to prejudice the information about the plant source. During demethoxylation, (i) simple cleavage of the aromatic C-0 bond or (ii) multistep reactions via O-CH3 bond scission (demethylation) are p o s ~ i b l e . ~If~the * ~ former ~ * ~ ~ is a major process, the generation of vanillyl residues from syringyl residues, and subsequently p-hydroxyphenyl residues from vanillyl residues can be expected as suggested by Niklas and Pratt.23 Consequently, at some point in the geological
-
0
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200
.-e0 m
K
n -I a
ii
0
a
!
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I
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Figure 2. Cunonia capemis heated a t 200 "C. (a) SyringylJ
Ohta and Venkatesan
274 Energy & Fuels, Vol. 6, No. 3, 1992 R
.I
Y
-1.0
I
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-I
h
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I
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.-
1( c )
/ P-ACDiP-ALD
0
c
m
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i
n
0
a
0.0
50
100
150
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Time (hr)
Figure 3. Pinus pinea heated at 200 OC. (a) Vanillyl and p -
hydroxyphenyl phenols concentration after pyrolysis. (b) First-order kinetics plot. (c) Acid/aldehyde ratio after pyrolysis. R = -CHO,-COCH3, or -COOH. column, the vanillyl phenol formed from the stepwise demethoxylation of syringyl phenol moiety would reflect the inputs of gymnosperms to a large extent, rather than the original angiosperms, which are relatively rich in syringyl phenols. Our data (Figure l),however, show no appreciable accumulation of vanillyl or p-hydroxyphenyl groups throughout the experimental period. In another pyrolysis experiment conducted with a gymnosperm wood P. pinea (Figure 3a), the total yield of p-hydroxyphenyl group (p-hydroxybenzaldehyde, p-hydroxyacetophenone, and p-hydroxybenzoic acid) decreased quickly with heating time and was below detection limit after 130 h, suggesting that the generation of p-hydroxyphenyl residue from vanillyl phenol moiety is minimal. These findings lead us to conclude that the simple aromatic C-O bond cleavage or stepwise demethoxylation is a minor process in thermal alteration of lignin polymer. The multistep reactions via 0-C bond cleavage resulting first in demethylation20*24 would probably be a dominant process during thermal demethoxylation (which could occur subsequent to dehydroxylation), although catechol-like intermediates identified in peat, lignite, and fossil wood ~ a m p l e were s ~ ~not ~ ~detected ~ in our alkaline CuO (32) Chaffee, A. L.; John, R. B.; Baerken, M. J.; deleeuw, J. W.; Schenck, P. A.; Boon, J. J. Org. Geochem. 1984.6, 409-416.
oxidation products from heated wood samples. This is because of the extremely labile nature of the catechol-type structures in alkaline solution.33 As a first step toward simulating geochemical lignin degradation, pyrolysis experiments were carried out with dry wood powder (with and without minerals) after solvent extraction and freeze-drying. However, this condition could be modified a little by carbohydrates, one of the major component of plant tissues used for pyrolysis. In nature, however, the carbohydrates are degraded by microbial processes rather than thermal processes and disappear much earlier than lignin phenol^.'^-^^ The polysaccharides could produce water via dehydration reaction during pyrolysis. Water droplets were, indeed, noticed on the inside glass wall of the heating tubes when cooled at the end of pyrolysis, although the amount of water thus generated could not be determined. This water should have been generated by dehydration mainly from cellulose and hemicellulose moieties and presumably might have affected lignin degradation as well to some extent. The newly generated water might, indeed, have influenced the lignin degradation reactions, in a manner very similar to natural processes which proceed in the presence of some amount of water although we started off with dry pyrolysis. Hydrous pyrolysis, by adding known amount of water, was, therefore, not attempted. As indicated by an increase in the S/V ratio at the first pyrolysis stage (Figure 2a), the vanillyl residues in C. capensis ligninsare more susceptible to degradation reactions including demethoxylation than syringyl residues. However, the slope of kinetic plots for V-TOT in P. pinea (Figure 3b) is lees steep than those for V-TOT and S-TOT in C. capensis in the first stage (Figure l), suggesting vanillyl residues in P. pima are more resistant to thermal degradation than either vanillyl or syringyl residues in C. capensis. Apparently, the thermal stability of vanillyl and syringyl phenols depend on the nature of matrix within which they are embedded. In short, the relative stability of the two residues is probably influenced by the lignin source itself. This is illustrated further by the following additional examples of pyrolyses experiments of pigmy date palm (P. roebelenii) and a humic acid sample from peats. The syringyl residues in these two samples were degraded more quickly than coexisting vanillyl residues, resulting in a drop of the S/V ratio (Figure 4a) in contrast to C. capensis (Figure 2b). The two examples, as well as C. capensis, showed that a relatively rapid degradation of one of the two lignin residues (vanillyl or syringyl) occurs in early pyrolysis stages and a very slow decomposition of both the residues almost reaching a plateau in later stages. Water from carbohydrate dehydration could influence lignin degradation (as discussed previously) not only at early and late stages but throughout the entire pyrolysis process because water droplets were still present after the wood specimen was pyrolyzed for 250 h (finalheating time interval). However, humic acid (which contains no carbohydrates) also exhibits two reaction rates (Figure 4b). This lends support to the two-stage lignin degradation from the wood samples and further implies that the two rates observed are not artifacts introduced by carbohydrates and that the water generated by carbohydrate dehydration, apparently, does not significantly influence the rates of lignin degradation. One of the prominent changes in lignin phenol composition on pyrolysis was an increase in the ACD/ALD ratios with increasing heating time. The ratio of vanillic acid to (33) Varagnat, J. In Kirk-0th". Encyclopedia of Chemical Technology, 3rd ed.; Wiley Interscience: New York,1981; Vol. 13, pp 39-69.
:
Energy & Fuels, Vol. 6, No. 3, 1992 275
Pyrolysis of Wood Specimens
.-0
c
m
K
? v)
: O
Phoenix roebelenii
.
HumicL Acid
1 from Peat
0'4 0 0.0
0
q
without minerals described earlier (Figures 1 and 3b), exhibiting a sharp degradation at the first stage, followed by a slow decline of lignin phenols yield at the second stage. Except for acidic montmorillonite (Mo-KlO), the kinetic plots of other minerals examined (Figure 5) are almost superimposable on the plots for the pyrolysis of C. capensis without minerals (Figure 1) and therefore seem inactive as catalysts on lignin degradation. Only Mo-K10 accelerated significantly the pyrolytic degradation of lignin polymer. The slopes of the plots for V-TOT and S-TOT are much steeper at the first stage than those of the corresponding ones in Figure 1,which indicates that the degradation rates of lignin phenolic residues are greatly accelerated by this mineral. About 94 and 93% of V-TOT and S-TOT in original lignins, respectively, are degraded during this stage. This marked catalytic action of acidic clay mineral is in agreement with the findings of Hayatsu et al.26. Slopes of the plots at the second stage are less steep than those with other minerals as well as in the runs without minerals. Benzoic acid and 3-hydroxy- and 3,5-dihydroxybenzoicacids were generated during this stage, which were not detected in other pyrolysates. These simple benzoic acids increased in content with heating time. They could be produced from nonlignin wood components such as cellulose, hemicellulose, and terpenic and phenolic lipids. Their negative correlation with lignin phenols yield here implies lignin polymer to be possibly one among the several precursors. A n increase in S/V ratio was commonly observed at the first pyrolysis stage in all experimental runs with and without minerals (Figures 6a and 2a). The magnitude of this increase varied among mineral species examined. In all the runs the S/V ratios approached a plateau at later stages of pyrolysis, i.e., to constant value between 5.0 and 6.5, and around 7.5 in the cases with and without minerals, respectively. Thus, the S/V ratio in C. capensis lignins was thermally modified less than twice as much as original value (4.3 for the not heated specimen). The S/V ratios in P. roebelenii and humic acid samples decreased in early pyrolysis stages to about 2/3 of the original values and was held constant at later stages (Figure 4a), exhibiting trends almost like a mirror image of those in C. capensis lignins. Microbial degradation is known to modify, to some extent, the S/V ratios of lignins in plants during early diagene~is."-'~J' The S/V ratios modified once microbially would be again altered thermally at relatively early stages of geochemical processes, when the majority of lignin phenol residues might be degraded quickly. Our experimental results suggest further that some portions of both vanillyl and syringyl phenol residues of lignins in plants, which are refractory enough to escape the quick thermal degradation, could survive diagenetic and early catagenetic processes with a little modification of the S/V ratios to provide not quantitative but qualitative information about the original plant source. This observation suggests that S/V ratios could be used qualitatively as a paleovegetation index and in turn, a crude paleoclimatic marker. Figure 6b shows the trends of the ACD/ALD ratios in runs with Mo-K10 and montmorillonite. The data for other runs, as well as the run without minerals (Figure 2b), lie between those of the two samples in Figure 6b. Except for the Mo-K10, the other minerals examined did not accelerate reactions resulting in high ACD/ALD ratios. However, an increase in phenolic acid yield with heating time is a common trend to all experimental runs with and without minerals. Although Benner et ala1*did not observe measurable increase in the ACD/ALD ratios in any of these phenol families in their investigation of very early
50
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-0O 5O \
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Time (hr) Figure 4. (a) Syringyl/vanillylphenols ratio after pyrolysis of Phoenix roebelenii and humic acid from Kerry (Ireland) peat. (b) Kerry peat humic acid heated at 200 "C,fist-order kinetics plot of total (V-TOT+ S-TOT)phenols. vanillin (V-ACD/V-ALD) and that of syringic acid to syringaldehyde (S-ACD/S-ALD) in C. capensis increased rapidly at the first pyrolysis stage and gently in the second stage (Figure 2b). For example, in the first 60 h of heating (first stage), the yield of aldehydes is reduced by 80-90% while that of acids by 15-40%. A t the end of 200 h of pyrolysis, the decrease in the yield of aldehydes amounts to 290% while that of acid is only about 2040%. Apparently, the increase in ACD/ALD ratios is driven largely by the decrease of aldehydes. Similar trends were observed for vanillyl and p-hydroxyphenyl residues in P. pinea (Figure 3c), and for all the three phenolic residues in the P. roebelenii and peat humic acid samples. The bonds from Ca and CP in lignin side chains are known to be easily cleaved by microbial Our findings indicate that the bonds around the a-carbon in the lignin side chains are affected thermally also. In addition, the thermal cleavage of the 8-0-4 bonds could modify the a- and 8carbon bonding too. For example, a double bond may be introduced after the 8-0-4 cleavage, resulting eventually in an increase in ACD/ALD ratio. Caution should, therefore, be exercised in interpreting ACD/ALD ratio, considering that thermal (diagenetic) changes could influence these ratios besides microbial degradation. The thermal degradation of gymnosperm lignins was found to be accelerated by clay minerals, particularly, by acidic clay minerals.26 To explore the catalytic behavior of minerals on angiosperm lignins, C. capensis was pyrolyzed with neutral (montmorillonite and kaolinite) and acidic clay minerals (Montmorillonite KlO), and also common minerals associated with coals (i.e., pyrite and marcasite). Figure 5 illustrates the first-order kinetic plots whose profiles resemble those of the pyrolysis experiments (34) Saiz-Jimenez, C.; deleeuw, J. W. Org. Geochem. 1984,6,417-422. (35) Ertel, J. R.; Hedges, J. I. Geochim. Cosmochim. Acta 1984, 48, 2065-2074. (36) Hedges, J. I.; Blanchette, R. A.; Weliky, K.; Devol, A. H.Geochim. Cosmochim. Acta 1988,52, 2717-2726.
Ohta and Venkatesan
276 Energy & Fuels, Vol. 6, No. 3, 1992
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Figure 5 . Cunonia capensis heated at 200 "C with minerals. First-order kinetics plob. Ka, kaolinite; Py, pyrite; Ma, marcasite; Mo, montmorillonite; Mo-K10, acidic montmorillonite.
geological samples, therefore, represent probably an integrated signal of microbial and thermal effects, and possibly even leaching effects.
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Figure 6. Cunonia capensis heated a t 200 "C with minerals. (a) Syringyl/vanillyl phenols ratio. (b) Acid/aldehyde ratio. For explanation of abbreviations, see Figure 5.
diagenetic trends of mangrove leaves, they noticed high ratios for vanillyl phenols in the leachate. This suggests that the lignin phenols composition could be influenced by solubility besides microbial degradation in early diagenesis. The ACD/ALD ratios in oxidation products of
Conclusions A series of pyrolysis experiments conducted with wood specimen led to the following observations regarding thermal degradations or alterations of lignin polymer. 1. The yields of lignin phenolic residues after thermal degradation can be described by the first-order kinetics. Thermal (and diagenetic) reactions can be significantly accelerated by acidic clay minerals. 2. Simple aromatic C-0 bond cleavage resulting in demethoxylation of lignin phenolic residues seems unlikely. 3. The syringyl/vanillyl phenol (S/V) ratios in original lignins can be thermally modified. Yet, the ratios remain constant at later stages of pyrolytic processes indicating that the S/V ratios can be used as a qualitative index of vegetation type in geological samples. This also implies that the ratio, in tum, is useful as a paleoclimatic signature to a limited extent. However, this hypothesis remains to be tested with geological samples of known floral history at different diagenetic stages. Further, the results should also be useful in the study of thermal histories through analysis of lignin remains in sedimentary rocks that have been exposed to mild heating. 4. The bondings at a-carbon in side chains are thermally unstable. Thus the phenolic acid content in CuO oxidation products increased with increasing pyrolysis time. Thus ACD/ALD ratios are affected by both microbial oxidation as well as (thermal) diagenesis. 5. Some of the observed reaction rates also may eventually be revealing of the types and ultrastructures of different lignins in fresh plant tissues. 6. The data help define the changes in the lignin compositional trends that characterize the early stages of coalification. Acknowledgment. We thank Dr.B. Prigge for the wood samples (and for their identification), Dr. E. Idiz for collecting Ireland peat samples, Mr. J. Hassanzadeh for pyrite and marcasite, Dr. J. I. Hedges for Lake Washington
Energy & Fuels 1992,6, 277-286 mud, data, and discussions pertaining to lignin phenols determination, Dr. S . Steinberg for contributing to preliminary data, Mr. E. Ruth for GC/MS data of lignin phenols, Miss R. Pourvasei for her meticulous technical assistance, and Mrs. R. Pichay for graphics and word processing. Critiques from two anonymous reviewers and Prof. I. R. Kaplan are appreciated. This project was
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funded by NSF Grant No. EAR-8816410 to M.I.V. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (to M.I.V.). Preliminary data from this work was presented at the 200th ACS Meeting in Washington, DC, August 26-31, 1990.
Laboratory-Scale Coal Combustor for Flue Gas Emission Studies Harvey G. Stenger, Jr.* and Eduardo C. Meyer Department of Chemical Engineering, Iacocca Hall 111, Lehigh University, Bethlehem, Pennsylvania 18015 Received November 7, 1991. Revised Manuscript Received February 7, 1992
A laboratory-scale fluidized bed combustor is described, which is used to produce actual flue gas for emission studies. The combustor operates at 80e935 OC with continuous coal feed of 0.03-0.12 kg/h, fluidization velocities of 0.2-0.6 m/s, and gas flow rates in the range of 5-15 L(STP)/min. The combustion facility is fully automated through a computer for control and data acquisition. Sulfur dioxide, NO, CO, C02,CHI, NH3, NzO, and HzO, present in the flue gas, are rapidly analyzed and monitored using a computer-controlled Fourier transform infrared spectrometer with a 10 m path gas cell. The operability of the combustor was tested for various temperatures, coal feed rates, and air flow rates, and its efficiency computed to be between 50 and 75%. To examine the combustion system's ability to evaluate flue gas emission abatement methods, SO2 was removed from the exhaust gas by adsorption on mordenite. The mordenite was found to have only a fraction of the sorbent capacity reported in other studies with simulated flue gas; this capacity discrepancy is explained by the coadsorption of other components from the real flue gas.
Introduction The development of new flue gas clean up technologies has been hindered by the difficulties in taking meaningful measurements in laboratory-scale apparati. Technologies such as semipermeable membranes,' regenerable sorbents: selective catalytic reduction: electrochemical reduction: direct decomp~sition,~ and others require testing using actual flue gas for extended periods of time. Using actual flue gas ensures that the materials of the technology (membranes, catalysts,sorbents, electrodes, etc.) are tested in the presence of the complete spectrum of components anticipated in a power plant. These components include Nz,Oz, COz, H20, CO, suspended particulate matter (SPM), volatile organic compounds (VOC), NO, NO2,SOz, ~~
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(1) Majumdar, S.;Cha, J. S.; Papadopoulos, T. H.; Sirkar, K. K.; Kim, S.S. A novel liquid membrane technique for removal of sulfur dioxide/ nitrogen oxide (NOx) from flue gas. Repr.-Am. Chem. SOC.,Diu. Pet. Chem. 1991,36 (l),25-32. (2) Jozewicz, W., Chang, J. C. S.,Sedman, C. B. Bench-scale Evaluation of Calcium Sorbents for Acid Gas Emission Control. Enuiron. 1990, 9,137. (3) Beeckman, J. W.; Hegedus, L. L. Design of Monolith Catalysts for Power Plant Nitrogen Oxide (NO+) Emission Control. Ind. Eng. Chem. Res. 1991, 30, 969-78. (4) McHenry, D.; Winnick, J. New matrix materials for molten electrolyte membranes. Sep. Sci. Technol. 1990,Z (13-15), 1523-35. (5) Li, Yuejin; Hall, W. K. Catalytic Decomposition of Nitric Oxide over Cu-Zeolites. J . Catal. 1991, 129, 202-215.
SO3,H2S04,HC1, metal oxides such as arsenic oxide, NzO, and others that may be too small to measure. Because of operational simplicity many new technologies are tested using simulated flue gases made by blending several gases together. An accurate and cost effective method of blending up to 15 Components, some in trace amounts, is difficult to construct. This causes the simulated flue gas to be lacking in its representation of actual flue gas; thus the testing of new technologies cannot be considered completely rigorous. Yang et al.? in studying SCR catalyst poisoning, have attempted to overcome this problem by intentionally adding suspected poison metals to the catalyst during its preparation. While this method is a simple procedure to obtain poisoning data rapidly, it may provide misleading results, since poisoning a catalyst (or sorbent or membrane, etc.) during preparation will not be chemically similar to that when it is contacted by a gas-phase species. The importance of trace impurities on catalyst deactivation studies is illustrated by the following example. A SCR catalyst having a density of 900 kg/m3 operated at a 25000 h-' space velocity (at STP) will contact 1.24 mol of flue gas per gram of catalyst per hour. If the flue gas (6) Buzanowski, M. A.; Yang, R. T. Simple Design of Monolith Reactor for Selective Catalytic Reduction of Nitric Oxide for Power Plant Emission Control. Ind. Eng. Chem. Res. 1990, 29, 2074-8.
0 1992 American Chemical Society 08~7-0624/92/2506-0277$03.QQ~Q
