844
Ind. Eng.
Chem.Process Des. Dev.
Suuberg. E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. D e v . 1078, 17, 37. Suuberg. E. M.;Peters, W. A.; Howard, J. 8. “Proceedings, Seventeenth Svmnoslum (International) on Combustion”: The Combustion Institute: P k h r g h , 1979; p 117. ’ Thurner, F.; Mann, U. Ind. Eng. Chem. Process Des. Dev. 1981. 20, 482. Vlllermaux, J.; Antokre, 8.; Lede, J.; Soullgnac, F. Prep., D/v. Fuel Chem., Am. Chem. Soc. 1083. 28(5),390.
1985, 2 4 , 844-852
Wenzl, H. F. J. “The Chemical Technology of Wood”: Academlc Press: New York, 1970.
Received for review August 2, 1982 Revised manuscript received September 19, 1984 Accepted October 13, 1984
Product Compositions and Kinetics in the Rapid Pyrolysis of MiHed Wood Lignin Theodore R. Nunn, Jack B. Howard, John P. Longwell, and Willlam A. Peters’ Energy LaboratOIy and Department of Chemlcai Engineering, Massachusetts Institute of Technobgy, Cambrldge, Massachusetts 02139
Yields, compositions,and rates of evolution of major products from batch pyrolysis of predried milled wood lignin from sweet gum hardwoodwere measured for the temperature range 600-1400 K under 5 psig of helkrm, at heating rates and Umes at final temperature of 1000 K/s and 0 s, respectively. Approximately 100-mg layers of 1100 km thick Hgnin flakes, thinly spread on a hot stage, were heated so that volatiles residence times at elevated temperatures were minimized. Total weight loss Increased strongly with temperature to an asymptote of 88% at 1050 K. Tar is the major pyrolysis product above 800 K. Its yield exhibits a maximum of 53 wt % at 900 K and declines to 47 wt % at 1150 K. The latter represents 57% of the energy of the dry lignin. Secondary cracking of the tar contributes to the yields of CO, CH,, C2H4,COP, and other light gases above 950 K. Carbon monoxide dominates the gas yield above 850 K and attains a yield of 19 wt % at 1440 K. A single reaction
flrst-order decomposition model describes well the global rates of evolution of most major products, except tar. However, the data imply that most products are evolved by more complex reaction pathways.
Introduction The pyrolysis of wood, cellulose, and other biomass materials has been studied extensively. Previous work, including several kinetic investigations, has been reviewed for cellulose by Hajaligol(1980),Lewellen et al. (1977), and Molton and Demmitt (1977), for wood by Roberta (1970) and Wenzl (1970), and for different forms of biomass by Diebold (1981), Antal (1980), Milne (19791, and Peters (1978). Lignin is a major constituent of all woods (typically 15-30 wt 5%) and of many other biomass materials. The chemical structure of naturally occurring lignin varies with biomass type. It is generally agreed, however, that most lignins are complex copolymers of primarily three distinct phenyl propane monomers. Degradation of lignin to recover valuable single-ring aromatic chemicals has therefore commanded interest, especially in the forest products industry where lignin is a major byproduct in pulp and paper manufacture. Work on lignin degradation to phenols by oxidative and hydrolytic decomposition (1971) and by liquid phase catalytic hydrogenation (1966) has been reviewed by Goheen. Pyrolytic pathways are also of interest and previous work has been discussed by Klein (19811, Klein and Virk (1981), Iatridis and Gavalas (1979), Allan and Matilla (1971), Goheen (1966,1970, and Wenzl(1970). Many of the previous studies involved slow heating of large samples under conditions where secondary reactions of the volatiles contributed to the observed product spectrum. Furthermore, large variations exist in reported product distributions. These may arise from different reaction conditions and from the use of different types of lignin. For example, Kraft lignin undergoes much more severe chemical modification during preparation than does milled wood (Bjorkman) lignin, which more closely reaembles the lignin 019&4305/85~1124-0844$01.50/0
in the parent wood (Pearl, 1967; Kollman and Cote, 1968). Iatridis and Gavalas (1979) investigated the effect of temperature and sample residence time on yields of tar, char, light gases, and single ring phenols from the pyrolysis of powdered Douglas fir precipitated Kraft lignin, using an electrical screen heater reactor similar to the one employed in the present work. They report a total volatiles yield of around 60 wt % at 923 K and 120 s solids residence time, including 4.8 wt 5% CH,, -3 wt % CH,OH, 9.2% CO, and 3-4 wt % light (single ring) phenolic compounds. Kraft lignin pyrolysis has also been studied in a helium microwave plasma by Graef et al. (1980), who report volatile yields above 65 wt % including 54 w t % as a gas rich in CO, H,, and C2H2. A great deal remains to be learned about lignin pyrolysis. There is need to determine the independent effects of reaction conditions such as temperature, heating rate, solids residence time, volatiles residence time, pressure, gaseous atmosphere, and sample dimension on the yields, compositions, and rates of production of pyrolysis gases, liquids, and chars. Quantitative information on the separate contributions to pyrolysis of primary decomposition and of volatiles secondary reactions is also lacking, and we are unaware of any systematic studies of the rapid pyrolysis behavior of milled wood lignin. These details are needed to advance basic understanding of lignin pyrolysis and to provide models capable of predicting the thermal chemical reaction behavior of lignin and lignocellulosic materials. Also of interest is a determination of the extent to which the rapid pyrolysis behavior of whole biomass can be predicted from corresonding information on the pyrolysis behavior of ita three major constituents (cellulose, hemicellulose, and lignin). In response to these needs, the present paper presents results on the effect of temperature on the yields, com0 1985 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 845
positions, and rates of evolution of major products from the rapid pyrolysis of milled wood lignin from sweet gum hardwood (Liquidambar styraciflua), under 5 psig of helium. The work is part of an ongoing program aimed at providing better quantitative understanding of the rapid thermal decomposition behavior of whole biomass and of its three major constituents. Previous communications (Hajaligol, 1980; Hajaligol et al., 1982; Nunn, 1981; Nunn et al., 1985) present results on filter paper cellulose and sweet gum hardwood, and a subsequent paper (Ghosh et al., 1985) will discuss our findings on one hemicellulose (xylan). Because acquisition of data and kinetic parameters pertinent to the whole biomass simulation question was an objective, the lignin was prepared from the same master sample of sweet gum as the powdered wood studied by Nunn et al. (1985). The milled wood form was chosen to provide a reasonable representation of the lignin as it occurs in the wood.
Experimental Section Reactor Description. The measurements were performed in a captive sample electrical screen heater reactor described in detail previously (Hajaligol et al., 1982). All products except H2 are collected to allow direct measurement of material and elemental balances. Timetemperature histories of the pyrolyzing lignin are recorded to allow kinetic parameters to be derived from laboratory data on integral product yields. Freshly formed volatiles rapidly exit the hot region immediately surrounding the decomposing sample, so that their secondary reactions are minimized (but not totally eliminated), and the primary decomposition of the biomass material is more closely interrogated. This reactor also allows the sometimes collaborative effects on pyrolysis of heating rate, substrate residence time, substrate dimension, and substrate temperature to be independently studied. Experimental Procedure. Approximately 40 g of powdered milled wood lignin was extracted from sweet gum hardwood especially for this study, in the laboratory of Professor H.-M. Chang of North Carolina State University using standard procedures. Heat transfer calculations (Nunn, 1981) show that for heating rates Ilo00 K/s major temperature gradients across the sample are eliminated when its critical dimension is below -100 pm. Attempts to prepare a 45-88-pm size fraction of lignin powder by dry sieving were unsuccessful because of excessive lignin losses and apparent clogging of the sieve openings. Small (1100 Fm thick) flakes were prepared by pressing -20-mg lots of the powder between two 2.5 X 15 cm parallel plates mounted between the jaws of a hand-operated catalyst pelletizing press. The flakes adhered to the parallel plates and had to be chipped off with a microspatula. During this chipping process, most of the 10-mm diameter flakes broke up into fragments, too small for the screen heater grid. Several pressings were therefore needed to generate enough acceptable flakes for the pyrolysis experiments. Although tedious, this technique allowed more efficient utilization of the lignin than sieving. The flakes were dried over desicant for at least 4 weeks prior to use. Their elemental composition is shown in Table I. Approximately 100-mg lignin portions were batched pyrolyzed by heating under 5 psig of helium at rates of 1000 K/s to preselected final temperatures between 600 and 1400 K. Sample cooling, at an average nominal rate of 200 K/s, was then immediately begun, with no holding time spent at the peak temperature. All pyrolysis products except H2were collected. Gaseous and light liquid prod-
Table I. Elemental Composition of Tars and Chars from Pyrolysis of Milled Wood Lignin % of element tema. K yield, wt % C H 0" milled wood lignin 59.1 6.0 32.0 tar 770 34.4 54.2 5.4 30.5 tar 970 52.6 59.9 5.5 tar 1020 51.4 62.0 5.2 21.9 tar 1440 43.1 62.1 5.4 24.0 char 580 96.9 59.8 5.8 29.4 char 800 62.1 5.5 29.0 50 char 1350 14.5 91.3 a Oxygen elemental analysis obtained from a Coulometrics carbon dioxide coulometer (Raines, 1981).
r
TAR
0 600
800 1000 1200 1400 PEAK T E M P E R A T U R E , K
Figure 1. Effect of peak temperature on yields of char (a),tar (A), and gas (including water) ( O ) , from pyrolysis of milled wood lignin flakes. [Points: experimental data; curves: trendlines; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; flake thickness S 100 pm].
ucta were analyzed by gas chromatography. Char yield is operationally defined as the amount of material remaining on the screen after a run is concluded, and is determined gravimetrically. Since the reactor provides essentially uniform heating of the entire sample, char so defined is believed to reflect chemical modification of the entire substrate and is therefore taken as a legitimate reaction product not a mixture that includes some unreacted lignin. Tar yield was also determined gravimetrically (see below), and both tar and char were further characterized by elemental analysis. The products collection, recovery, and analysis procedures were similar to those described previously (Hajaligol et al., 19821, but certain modifications described by Nunn (1981) were employed to improve the precision in the present data. Good material and C and H elemental balances (5-10% closure) were generally obtained (see Table 11). Results Product Yields and Volatile8 Compositions. Yields are expressed in percent by weight of the dry lignin and plotted as a function of peak temperature. The findings are summarized in Figures 1-11, where the points show the laboratory data and the curves are free-drawn trend lines. Figure 1shows the yields of char, tar,and gas (including water). Under the present conditions, decomposition of the lignin is first observed at about 600 K. Sample weight loss increases with temperature until 86 wt % of the lignin is converted to volatile material at about 1050 K. Above this temperature, the char yield remains constant at 14 wt 9%. Most of the sample weight loss occurs between 700 and 900 K. Tar is operationally defined as recoverable liquid product: (a) condensed within the reactor vessel and (b) remaining within a glass wool trap downstream of the
846
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 5
2
>
..LAJ
!
I
I
1
o
>
600 800 1000 1200 1400 PEAK TEMPERATURE, K
J
> I+ n LL
0
"1 25
0 1 400
600
800 1000 1200 1400
PEAK
Figure 2. Effect of peak temperature on the yield of carbon monoxide from pyrolysis of milled wood lignin flakes. [Points: experimental data, curve: trendline; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 pm].
" -
,
50
TEMPERATURE, K
Figure 5. Effect of peak temperature on the yield of water from pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 pm].
i.-" i
CH4
-I
I
P
; I w
3 m >
1
z
n 0.2
n J
w
w>
tJ
Figure 3. Effect of peak temperature on the yield of methane from pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 p i g (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 pm]. I
I
I
I
Figure 6. Effect of peak temperature on the yield of ethylene from pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 gm].
f-
0.40r
J
0.35-
P
c
i
/"
i
LLl 2.51
I
I
1
I
1
I 1000
I 1200
1400
1
c2 H6
>
3.0
4,
I
600 800 1000 1200 1400 PEAK TEMPERATURE, K
600 800 1000 1200 1400 PEAK TEMPERATURE, K
I
,
I
o
0 0.30C LL
c
0.25-
I
p w
0.200.15-
1.0
0.5 0
600 0
800
1000
1200
1400
PEAK TEMPERATURE, K
2
w
0.05 0
- 1 -
600
800
PEAK
TEMPERATURE, K
Figure 4. Effect of peak temperature on the yield of carbon dioxide from Pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 psig (helium); nominal heating rate = loo0 K/s; flake thickness S 100 pm].
Figure 7. Effect of peak temperature on the yield of ethane from pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 gm].
reactor after heating to 373 K. This material and the product gases are initially evolved in about equal yields (at -600 K), but tar production becomes much greater as the peak temperature is increased above 700 K. The lignin pyrolysis gas yield increases monotonically with temperature to an apparent asymptote of about 36 wt % at around 1150 K. As temperature increases, tar yield goes through a maximum of about 53 wt % a t 850-950 K and then declines to an essentially asymptotic yield of about 47 wt % at 1150 K, probably due to secondary cracking to light volatiles. The lignintar maxi" is somewhat broader and flatter than tar maxima previously reported for wood (Nunn et
al., 1985) and cellulose (Hajaligol et al., 1982) pyrolysis. The structures of these three substrates and elemental analyses of their pyrolysis tars (Table I; Hajaligol et al., 1982; Nunn et al., 1985) imply that lignin tar is the most aromatic and would therefore be expected to have greater thermal stability than cellulose and wood tar. The percentage reductions in tar from the maximum yield to the yield observed at the highest peak temperature studied are 20, 20, and 15, respectively, for cellulose (Hajaligol et al., 1982), sweet gum hardwood (Nunn et al., 1985),and lignin (Figure l), lending support to this picture. Within the data scatter, tar yield appears constant above 1150 K. One possible explanation of this approximate
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 847 Table 11. Elemental, Total Mass, and Energy approx "ultimate" component" yield," wt % lignin char 14.0 tar 47.0 co 19.0 3.2 CHI 4.1 coz 0.9 C2H4 0.3 CZH, 3.8 H2O 1.4 HCHO 0.3 Cab CHaOH 1.7 0.9 CHSCHO 0.6 butene + ethanol acetone furan 0.3 acetic acid 0.2 0.2 misc. oxygenates
Balances for Pyrolysis of Milled Wood Lignin C 59.1 13.3 31.1 8.1 2.4 1.1 0.8 0.2 0.8 0.26 0.6 0.5 0.3 0.2 0.1 0.18
+
total
97.9
closure! %
98
heat of combust.? Btu/lb 10430 15000 12580 4340 23860 0 21630 22300 0 8190 21000 9770 11400 12780 13280 6270 18020
g of element/100 g of lignin
60.0
101
H 6.0 0.3 3.5
0 32.0 0.4 12.4 10.9
0.8 3.0
0.1 0.1 0.4 0.1 0.04 0.2 0.1 0.1
3.4 0.5 0.9 0.3 0.2 0.1 0.1
0.02
5.8 96
% of lignin
approx peak temp: K
energy in component 100.0 20.1 56.7 7.9 7.3 0 1.9 0.6 0 1.1 0.6 1.6 1.0 0.7 0.4 0.1 0.3
32.2
1050 1150 1440d 1440d 1440d 1440d 1100 900 900 1100 900 900 e e e e
100.3
101
100
a Meaning that for the present reaction conditions the observed yield remains essentially constant with further temperature increases. *All heats of combustion are from CRC (1976),except for lignin, char, and tar which are calculated from eq 1 after Mason and Gandhi (1980). CApproximate temperature at which the indicated product yield is observed. dYield still increasing as temperature increases. eInsufficient data to determine. f High-temperature yields of molecular hydrogen were estimated to be C0.25 w t % and were ignored in this analysis.
f
0.301
c3n6
O
i
t 4 0.05
w*
0 600
800 PEAK
1000
1200
1400
TEMPERATURE, K
0,5 o
600
800
Figure 8. Effect of peak temperature on the yield of propylene from pyrolysis of milled wood lignin flakes. [Points: experimental data; curve: trendline; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; flake thickness 5 100 pm].
$
1000 1200 1400
TEMPERATURE, K
PEAK
Figure 10. Effect of peak temperature on the yield of methanol from pyrolysis of milled wood lignin flakes. [Points: experimental data, curve: trendline; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; flake thickness 5 100 pm].
950
600
800
PEAK
1000
1200
1400
TEMPERATURE. K
600
BOO
PEAK
1000
1200
1400
TEMPERATURE,K
Figure 9. Effect of peak temperature on the yield of formaldehyde from pyrolysis of milled wood lignin flakes. [Pointe: experimental data; curve: trendline; pressure = 6 pig (helium); nominal heating rate = lo00 K/s; flake thickness S 100 pm].
Figure 11. Effect of peak temperature on the yield of acetaldehyde from pyrolysis of milled wood lignin flakes. [Points: experimental data, curve: trendline; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flakd thickness 5 100 pm].
asymptote in high temperature tar yield is that a major portion of the tar is very stable to thermal cracking under the present conditions. Previous studies of biomass pyrolysis (Wnezl, 1970; Stamm and Harris,1953),have shown
that product tars can contain bomponents of significantly different thermal stability, and similar behavior has been reported more recently for coal (Serio, 1984;Serio et al., 1983, 1984). An alternative view, however, is that this
848
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
asymptote merely represents that portion of the tar which was formed and escaped the neighborhood of the screen before the temperature of that region became high enough for secondary reactions. Further discussion of these issues is found in the preceding paper (Nunn et al., 1985). Some additional secondary reactions of tar above 1150 K are indicated, however, since yields of CO, CHI, C02, and C2H4 continue to increase at temperatures up to 1440 K (see Figures 2,3,4, and 6). Char yields are determined very reliably with this equipment and remain constant above lo00 K, implying an attendent cessation of substrate weight loss. This suggests that tar cracking is the probable source of these increased gas yields although, because of their scatter, the tar yield data alone would not imply this. Figure 2 shows the effect of peak temperature on the yield of carbon monoxide (CO)from lignin pyrolysis. This product is first observed at about 750 K and its yield increases strongly with peak temperatures to 15 wt % at about lo00 K. Above this peak temperature, the CO yield rises less strongly with increasing temperature, to a nonasymptotic value of about 19 wt % at 1440 K. Since total weight loss from lignin seems to be constant above 1050 K, these observations suggest that CO is evolved from both the primary decomposition of lignin and from secondary cracking of its pyrolysis tars. The CO formed from 950 to 1100 K, the temperature range over which most of the tar decrease occurs, amounts to about 7 wt %. The decrease in tar over this range is approximately 8 wt % , which is consistent with the picture that most of the tar which decomposes generates CO. Methane (CH,) evolution (Figure 3) begins near 750 K, rises quite strongly with increasing peak temperature to a yield of -2.5 wt % at 1000 K, and then increases more weakly to a nonasymptotic make of 3.2 wt % at 1440 K. This is consistent with Iatridis and Gavalas’ (1979) observation of an asymptotic CHI yield of 4.8 wt % from the rapid pyrolysis of precipitated Kraft lignin at 923 K and 120 s solids residence time. The effect of peak temperature on the yield of carbon dioxide (C02)is shown in Figure 4. As with wood (Nunn et al., 1985),evolution of this product occurs below 600 K, i.e., at temperatures much lower than for either CH, or CO. However, unlike wood pyrolysis, where the yield of C02 becomes asymptotic when there is no further weight loss, the C02 yield from lignin increases from 3.6 wt % at lo00 K to 4.1 wt % at 1440 K, an increase of 17% by weight of carbon dioxide. As with CO, the continued evolution of CHI and C02 after the cessation of weight loss (i.e., above 1050 K) implies that secondary cracking of tar contributes to the high temperature production of these compounds. Figure 5 shows the yield curve for chemical (Le., nonmoisture) water. The data scatter is as much as f25% based on water yield probably because of chromatography difficulties (Cosway, 1981). Lack of data at temperatures below 600 K precludes identification of the threshold temperature for water production from lignin under the present reaction conditions. The yield increases monotonically with temperature to a plateau of 3.8 wt % at 900 K. Since about 80% of the water evolves below 700 K where secondary cracking of tar is believed to still be relatively minor, it seems probable that most chemical water is a primary product of lignin thermal degradation. Klein and Virk (1981) have predicted an ultimate water yield for long-residence-time low-temperature lignin pyrolysis of 6 wt % based on model compound studies. The differences may be due to the different reaction conditions of the two investigations.
Figure 6 presents the data for ethylene (C2H4) production from lignin pyrolysis. These data are similar in behavior to those of CO and CHI in that C2H4is first detected near 750 K and increases steadily with increasing temperature while exhibiting a noticable change in global generation rate at about lo00 K. Although the C2H4data are somewhat scattered due to the small absolute quantities produced, a yield increase of about 0.15% by weight of lignin between 1000 and 1440 K is discernible. The ethane yield curve is displayed in Figure 7. Ethane is first detected at 750-800 K. Its yield increases monotonically with temperature to an apparent asymptote of 0.29 wt % at 1000 K. Propylene production, shown in Figure 8, begins at a peak temperature near 800 K and increases strongly with temperature in the range 850-950 K, to an apparently asymptotic yield of 0.27 wt % at 1100 K. Yield data on several of the oxygenated compounds observed in lignin pyrolysis including ethanol (and butene), acetone and furan, acetic acid and miscellaneous liquid oxygenates, exhibited so much scatter that trends in yield with peak temperatures could not be established with confidence. High-temperature yields of these products were estimated, however, and are presented in Table 11. Yield data on three of the light oxygenated liquid products, formaldehyde, methanol, and acetaldehyde, did exhibit good reproducibility, however, and results for these compounds are shown in Figures 9,10, and 11, respectively. Formaldehyde (Figure 9) appears below 600 K and over 75% of its asymptotic yield of 1.4 wt % (attained at about 900 K) is evolved by 770 K, i.e., before significant secondary cracking of tar is expected. Methanol production (Figure 10) is first observed slightly below 600 K and reaches a plateau of 1.7 w t % near 900 K. This compares quite favorably with the asymptotic yield of 2.95 wt % at 923 K and a long holding time (120 s) obtained by Iatridis and Gavalas (1979) for precipitated Kraft lignin. Almost 60% of the CH30H yield is reached by 800 K. Acetaldehyde (Figure 11) production begins at 650 K, and attains an asymptotic make of 0.85 wt % by 900K. About 75% of its production is complete by around 770 K. Evolution of such large fractions of the ultimately observed yields of these products at temperatures below those required here for significant tar cracking (compare Figure 1)suggests that other pathways, perhaps including direct thermal decomposition of the lignin itself, play a major role in the generation of these products. Material, Elemental, and Energy Balances. Elemental analysis of the milled wood lignin and of product tars and chars generated at different peak temperatures are presented in Table I. The tar shows increasing carbon and an overall decline in oxygen content with increasing temperature, suggesting that oxygen functionalities in the tar are more thermally labile. Table I1 presents elemental, total mass, and energy balances for milled wood lignin pyrolysis. Since some of the light volatiles data are highly scattered, this analysis was performed using estimated ultimate yields for these compounds rather than the data from one specific experiment. The table includes estimates of the peak temperatures where the asymptotic yield, or highest observed yield, occurs, Since data on the separate yields of butene and ethanol, acetone and furan, and miscellaneous oxygenated compound fractions were unavailable, these compound groups were respectively assumed to be ethanol, acetone, and benzene, to allow their contributions to the elemental balances to be estimated. This approximation is reasonable because the total contribution of these com-
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 849
pounds to the ultimate product spectrum is only 1.1wt % , Since no high-temperature char analysis was available, the char was assumed to be 100% carbon. Heats of combustion for the individual gas products were obtained from CRC (1976) and the values for wood, char, and tar were calculated using the correlation of Mason and Gandhi (1980) Q = 146.58(C) + 568.78(H) - 51.53(0) (1) where Q is the gross heating value in Btu/lb on a dry basis and (C), (H), and (O), respectively, are the carbon, hydrogen, and oxygen contents in weight percent. The balances for total mass, and for carbon, hydrogen, and oxygen are good. The asymptotic energy balances show that char accounts for more than 20% of the parent lignin energy content. This reflects the larger yield of this product in contrast with wood pyrolysis, where only 12.1% of the parent wood energy value is accounted for by the asymptotic char yield (Nunn et al., 1985). The tar yield of 47 wt % at -1150 K accounts for almost 57% of the parent lignin energy content. Gases and light oxygenated liquids from hightemperature runs account for another 33%. Carbon monoxide and CHI account for more than half of the gaseous heating value presented in Table 11. Although the corresponding CO yield is about six times that of CH4,the latter accounts for almost as much of the energy of the lignin (7.3%) as does CO (7.9%). The close agreement between the sum total energy content of the individual pyrolysis products and the heating value of the parent lignin suggests that within the uncertainty of the present analysis, the high-temperature rapid thermal decomposition of milled wood lignin is approximately thermal neutral. Kinetic Modeling. Previous models of the high-temperature pyrolytic decomposition of coal, biomass, and other organic solids have ranged in descriptive and mathematical sophistication from simple unimolecular chemical models to complex analyses which incorporate physical transport and chemical reaction mechanisms (Howard, 1981a). The present data on yields of several individual products (except tar) were correlated with a single-reaction, firstorder decomposition model. This model is simple, has a history of successful utilization for condensed phase substances (Hajaligol et al., 1982; Franklin et al., 1981; Thurner and Mann, 1981; Suuberg, et al., 1978; Lewellen et al., 1977) and is useful in engineering calculations. For example, its application in calculations pertinent to pulverized coal ignition and flame stabilization is illustrated by Suuberg et al. (1979). In this model the rate of formation of a product i in yield Vi at time t is given by the expression d Vi _ - (Vi* - Vi) k,; exp dt
where k,. and Eiare respectively the preexponential factor and apparent activation energy for component i in the standard Arrhenius rate constant. The quantity Vi*is the ultimately attainable yield of product i, i.e., the yield at high temperature and long residence times. The other symbols have their usual meaning. Best fit values for the kinetic parameters kd,Ei, and Vi* (Le., those that minimize the sum of squared errors between calculated and observed yields) are obtained by comparing integral product yields Vi(T?,measured in nonisothermal experiments to different peak temperatures T’, and of overall duration t’, with corresponding values
Table 111. Kinetic Parameters for Pyrolysis of Milled Wood Lignin ~~
sroduct weight loss total gases
Ei, kcal/gmol
log kni, s d
V*. w t %
19.6 9.6 16.0 17.8 9.7 20.2 20.7 5.8 12.5 7.1 20.9
5.53 2.17 3.66 4.16 2.23 4.64 5.03 1.59 3.91 2.07 5.21
84.35 36.54 18.24 3.07 4.01 0.86 0.29 3.74 1.46 5.18 0.26
co
CH4
coz
CzH4 C2H6
H20 HCHO HzO + HCHO C3H6
std error of est,” wt % 5.76 1.85
1.00 0.18 0.26 0.07 0.03 0.31 0.21 0.39 0.03
“Defined as [Cj’rl(Vj,model - Vj,,,,a)*/(n - 3)]1/2, where n is the number of data points. 100 I
1
I
I
I
I
d
E 3
1
I
1
2o 0
600 800 1000 1200 1400
PEAK TEMPERATURE ,K
Figure 12. Experimental data (0) and model-generated curve for total weight loss from pyrolysis of milled wood lignin flakes. [Pressure = 5 psig (helium); nominal rate = 1000 K/s; flake thickness S 100 pm].
calculated using measured laboratory time-temperature histories, a nonlinear least-squares regression code, and the integrated form of eq 2
Vi* - Vi(T? Thus the fitting procedure takes account of contributions to conversion during both heatup and cooldown of the sample-i.e., t’ is the total time of the temperature program, not just the time to attain T’. The results for several products are shown in Table I11 along with the standard error of the fitting procedure. The tar evolution kinetics were not analyzed with this model because single-reaction first-order kinetics alone cannot predict a maximum in yield. Some of the light oxygenates were also omitted from the kinetic analysis because of large uncertainties in their yield data. The derived kinetic parameters were used to calculate curves showing yield as a function of peak temperature. The results for total weight loss, and for yields of gas, CO and CHI, are compared to the laboratory data in Figures 12, 13, 14, and 15, respectively. In general, the calculated curves fit the laboratory data quite well for temperatures up to about 1250-1300 K. Similar behavior was found for the other products listed in Table I11 (Nunn, 1981). If the high-temperature trend indicated in Figure 3 is correct, the single reaction model underpredicts the observed CHI yields (Figure 15) at temperatures above 1300 K. An improved correlation could probably be obtained by including an additional high-temperature decomposition pathway, and/or allowing for secondary decomposition of tar or other products. Implemention of the latter,
850
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 I
--r----T
I 1 1 1000 1200 1400
600
800
PEAK
TEMPERATURE, K
Figure 13. Experimental data (0) and model-generated curve for total gas production from pyrolysis of milled wood lignin flakes. [Pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 pm]. r
T
-r--r
co
000
1000 1200
800
PEAK TENPERATURE
1400
K
Figure 14. Experimental data (0) and model-gene ?d curve for carbon monoxide production from pyrolysis of milled wood lignin flakes. [Pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 pm] T
0
I
I
1
i-l
,
600 PEAK
800 1000 1200 1400 TEMPERATURE, K
Figure 15. Experimental data (0) and model-generated curve for methane production from pyrolysis of milled wood lignin flakes. [Pressure = 5 psig (helium); nominal heating rate = lo00 K/s; flake thickness 5 100 rm].
however, would require knowledge of the residence time distributions of volatiles in the high-temperature zones of the reactor. This information may be attainable by tracer studies and mathematical modeling, but the latter would not be easy (Hajaligol, 1980). The satisfying behavior of the single reaction model for most of the products and for most of the temperature ranges studied, and the generally low values for the standard error in the estimate (Table 111)show that the single-step, first-order kinetic model provides statistically satisfying correlations of the laboratory data. This model is thus a useful tool for comparing different data sets and performing engineering calculations. However, this does
not imply that these products are formed by simple mechanisms, since as noted above, the present yield vs. temperature data indicate an important role for multiple chemical pathways in the production of CO, CHI, C2H4, and other light gases, and in tar evolution at higher temperatures. Further, the activation energies and accompanying preexponential factors in Table 111are lower than those frequently reported for unimolecular thermal decomposition reactions (30-70 kcal/mol and 109-1022s-l, respectively; Suuberg et al., 1978). This behavior would be expected, however, if rate phenomena governed by a set of overlapping, independent parallel first-order reactions were approximated by a single first-order reaction (Jiintgen and Van Heek, 1970; Howard, 1981b). Interestingly, however, the activatiorl energy for total weight loss (19.6 kcal/g-mol) compares favorably with the value of 23.4 kcal/g-mol reported by Wenzl (1970) for lignin pyrolysis and falls within the range of values (17-38 kcal/g-mol) obtained by Domburg and Seergeeva (1969) for the pyrolysis of sulfuric acid lignins. Tang (1967) reported an activation energy and preexponential factor of 9 kcal/g-mol and 0.93 s-l, respectively, for lignin pyrolysis, which, for the temperature range 600-1440 K, imply first-order rate constants about two to four orders of magnitude lower than those predicted from the present parameters.
Discussion and Conclusions The following conclusions on the rapid pyrolysis behavior of milled wood lignin from sweet gum hardwood can be drawn from this study. (1)The captive sample electrical screen heator reactor is a reliable tool for determining product yields, compositions, and formation kinetics in the rapid pyrolysis of milled wood lignin under the present conditions. Good mass and elemental (C, H, and 0) balances have been achieved and product energy values are also readily estimated. (2) High conversions of the lignin to volatile products can be obtained in very short substrate residence times (
