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Ind. Eng. Chem.
Process Des. Dev. 1985, 2 4 , 836-844
main role. The fact that little difference in the liquefaction was observed between the results in decalin and 1methylnaphthalene also supports the direct process. On the other hand, the contribution of coal-derived materials which are more easily hydrogenated with Fe(CO), and serve as a hydrogen shuttler cannot be ruled out. Conclusion Fe(C0I5 acted as an excellent catalyst precursor for hydroliquefaction of Illinois No. 6 coal under the conditions of short contact time, high temperature (>460"C), and relatively low hydrogen partial pressure. Optimum conditions to obtain the maximum oil yield in the limited reaction time were 460 "C, 20 min or 480 "C, 10 min by using 1w t % (coal basis) Fe as Fe(C0)5at 80 kg/cm2 (cold) hydrogen. The active species derived from Fe(C0)5 is considered to serve in the direct hydrogen-transfer process
to the coal fragment radicals from molecular hydrogen. Literature Cited Chlen, P. L.; Sellers, G. M.; Weiler, S.W. Fuel Process. Technol. 1983, 7 , 1. Derbyshire, F. J.; Varghese, P.;Whkahurst, D. D.Fuel 1983, 62, 491. Eccles, R. M.; DeVaux, G. R.; Rakow, M. S. Pan-Pac. Synfuels Conf., Prepr. 1982, 388. Garg. D.;Givens, E. N. Fuel Process. Technol. 1983, 7 ,59. Mukherjee, D. K.; Chowdhury, P. B. Fuel 1976, 55, 4. Petrakis, L.; Grandy, D. W. Fuel 1980, 5 9 , 227. Petrakis. L.; Jones, G. L.; Grandy, D.W. Fuel 1983, 62, 671. Rosenthai, J. W.; Dahlberg, A. J.; Kuehler, C. W.; Cash, D.R.; Freedman, W. Fuel lS82, 67, 1045. Suzukl, T.; Yamada, 0.; Fujita, K.; Takegami, Y.; Watanabe. Y. Fuel 1984, 63,1708. Watanabe, Y.; Yamada, 0.; Fujita. K.; Takegami, Y.; Suzuki, T. Fuel 1884, 63,752. Zaczepinski, S. Pan-Pac. Synfuel Conf.. Prepr. 1982, 372.
Received for review January 5, 1984 Revised manuscript receiued November 15, 1984 Accepted November 28, 1984
Product Compositions and Kinetics in the Rapid Pyrolysis of Sweet Gum Hardwood Theodore R. Nunn, Jack B. Howard, John P. Longwell, and WlHlam A. Peters' Energy Laboratory and Department of Chemical Englneerlng, Massachusetts Institute of Technology, CambrMge Massachusetts 02 139
Yields, composition, and rates of evolution of major products from batch pyrolysls of predried sweet gum hardwood were measured for the temperature range 600-1400 K under 5 psig of helium, at heating rates and residence times at final temperature of 1000 K/s and 0 s, respectively. Approximately 100-mg layers of 45-88-pm wood powder, 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 93% at 1100 K. Tar is the major pyrolysis product above 800 K. I t exhibits a maximum yield of 55 wt % at 900 K and declines to an asymptote of 46 wt % at 1300 K. Secondary cracking of the tar is significant above 900 K and contributes substantially to the yields of CO, CH, C2H4,and other light gases. Carbon monoxide dominates the gas produced above 850 K and attains an asymptote of 17 wt % above 1300 K. A single-reaction first-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 kinetic pathways.
Introduction The pyrolysis of wood, cellulose, and other biomass materials has been studied extensively. Previous work, including several kinetic investigations, is reviewed by the following: cellulose, Molton and Demmitt (1977);Lewellen et al. (19771, and Hajaligol(1980);lignin, Allan and Mattila (1971), Klein (1981); wood, Roberts (1970) and Wenzl (1970);and different forms of biomass, Peters (1978),Milne (1979), and Antal(1980). There have been very few systematic studies of the independent effects of biomass type, 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 volatiles secondary reactions is also lacking. These details are needed to advance basic understanding of biomass thermal conversion pathways and to provide predictive models for existing and future processes for converting renewable resources to clean fuels and chemical feedstocks. A further application is in studies of flammability, flame spread, and related issues in fire research, because pyrolytic decomposition supplies the 0196-4305/85/1124-0836$01.50/0
volatiles that can ignite and support flaming combustion of condensed phase materials (Lewellen et al., 1977). This paper responds to some of the above information deficiencies. It presents recent results from a systematic study of the effect of temperature on the rapid pyrolysis of powdered sweet gum hardwood (Liquidambar styraciflua)under 5 psig of helium, including quantitative data on the yields, composition, and rates of evolution of major products. This work constitutes one fact of an ongoing research program aimed at providing better quantitative understanding of the rapid pyrolysis of whole biomass, and the three major biomass constituents-cellulose, hemicellulose, and lignin. Previous communications (Hajaligol, 1980; Hajaligol et al., 1982) have presented similar information on cellulose. A companion paper (Nunn et al., 1985) discusses our results on milled wood lignin extracted from the same parent sample of sweet gum used in the present study. A similar paper on one form of hemicellulose (xylan) is in preparation (Ghosh et al., 1985). Experimental Section Reactor Description. The measurements were performed in a captive sample electrical screen heater reactor 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 837
Table I. Elemental Composition of Tars and Chars from Pyrolysis of Sweet Gum Hardwood % of element feedstock or peak product temp,K yield,wt %" C H Ob sweet gum wood 49.5 6.1 44.6 tar I10 22.9 52.6 6.1 32.3 tar 895 52.5 53.9 5.9 37.1 tar 1355 50.2 55.0 6.2 32.3 char 610 91.0 50.1 6.2 42.2 char 810 53.2 51.5 6.1 39.7
,
201
I
I
I
I
1
n 18
83 l 6 b
14
c:
12
& z
10
8
d 6 w
Based on dry wood. Oxygen elemental analyses were obtained with a Coulometrics carbon dioxide coulometer (Raines, 1981). a
4
2 600 800 1000 1200 1400 PEAK T E M P E R A T U R E , K
6ot
401 20
0
600 800 1000 1200 1400 PEAK TEMPERATURE, K
Figure 1. Effect of peak temperature on yields of char ( O ) , tar (A), and gas (including water) (O), from pyrolysis of sweet gum hardwood powder. [Points: experimental data; curves: trendlines; pressure = 5 p i g (helium); nominal heating rat = lo00 K/s; powder size range = 45-88 pm.]
Figure 2. Effect of peak temperature on yield of carbon monoxide from pyrolysis of sweet gum hardwood powder. [Points: experimental data; curves: trendlines; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; powder size range = 45-88 pm.] 2.5
8n
'2 3
-
2.0
1.5 -
&
10-
ri W
0.5 -
s
>
I I 600 800 1000 1 2 0 0 1400 PEAK T E M P E R A T U R E , K
described in detail previously (Hajaligol et al., 1982). In
this equipment all produds except H2are collected to allow direct measurement of material and elemental balances. Time-temperature histories of the pyrolyzing wood are recorded to allow kinetic parameters to be derived from laboratory data on integral product yields. Further, freshly formed volatiles rapidly exit the hot region immediately surrounding the 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. Powdered sweet gum hardwood was dry sieved to give a 45-88 pm particle size range, and then dried over silica gel for at least 4 weeks. Its elemental composition is given in Table I. Approximately 100-mgportions of wood were batch pyrolyzed by heating under 5 psig of helium at rates of 1000 K/s to preselected final (peak) 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 H2 were collected. Gaseous and light liquid products were analyzed by gas chromatography. Char yield is operationally defined as the amount of material determined as the difference in weight between the screen plus unreacted wood, and the screen plus reacted wood. Since the reactor provides essentially uniform heating of the entire sample, any weight differences thus observed is believed to reflect chemical modification of the entire substrate, implying that char thus defined is a legitimate reaction product, not a mixture that includes some unreacted wood. Tar yield was also determined gravimetrically (Nunn et al., 1985) and both tar and char were further characterized by elemental analysis. The collection,
Figure 3. Effect of peak temperature on yield of methane from pyrolysis of sweet gum hardwood powder. [Points: experimental data, c w e s : trendlines: pressure = 5 psig (helium);nominal heating rate = 1000 K/s; powder size range = 45-88 pm.]
8 0 3
n
7.0 6.0
I
I
I
1
I
co2
8
o
-
-
5.0 -
-
-
-I
c
P
-
4.0
;'2.0 FJ, 1 I
I
1 .o
0
600 600
800 800
,
1000 1000
,
1200 1200
,
1400 1400
PEAK TEMPERATURC.K
Figure 4. Effect of peak temperature on yield of carbon dioxide from pyrolysis of sweet gum hardwood powder. [Points: experimental data; curves: trendlines: pressure = 5 psig (helium); nominal heating rate = 1000 K/s; powder size range = 45-88 pm.]
recovery, and analysis procedures were similar to those described previously (Hajaligol et al., 1982) but certain modifications described by Nunn (1981)were employed to improve the precision of the wood data. Material, and C and H elemental balances were usually closed to within f5-10% (see Table 11). Results and Discussion Product Yields and Volatiles Compositions. Yields are expressed as a percent by weight of the dry wood and plotted as a function of peak temperature. The results are summarized in Figures 1-9, where the points show the laboratory data and the curves are free-drawn trend lines.
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Table 11. Elemental, Total Mass, and Energy Balances for Pyrolysis of Sweet Gum Hardwood approx heat of % of wood g of element/100 g of wood “ultimate” combustion,b energy in component yield,” w t % C H 0 Btu/lb component 100.0 49.5 6.1 44.6 8450 wood 14660 12.1 char 7.0 7.0 tar 46.0 26.0 3.7 16.3 11470 62.4 co 17.0 7.3 9.7 4340 8.7 23860 6.5 CHI 2.3d 1.7 0.6 4.4 0 0 co2 6.1 1.7 C2H4 1.3d 1.1 0.2 21630 3.3 22300 0.5 C2H6 0.2 0.16 0.04 5.1 0.6 4.5 0 0 HZO 2.0 0.8 0.1 1.1 8190 1.9 HCHO CSHE 0.4 0.3 0.1 21000 1.0 1.7 CHSOH 1.5 0.6 0.2 0.7 9770 CHSCHO 1.4 0.8 0.1 0.5 11400 1.9 0.6 0.3 0.1 0.2 12780 0.9 butene + ethanol acetone + furan 0.9 0.6 0.1 0.2 13280 1.4 acetic acid 1.5 0.6 0.1 0.8 6270 1.1 misc. oxygenates 0.7 0.6 0.1 18020 1.4 total
94.0
closure,’ %
94
49.5 100
5.9 97
38.6
104.8
86
105
approx peak temp: K loo0 1300 1200 1520d 950 1520d 950 900 900 950 e 900 e
e e e
a Meaning that for the present reaction conditions the observed yield remains essentially constant with further temperature increases. bAll heats of combustion are from CRC (1976), except for wood, char, and tar which are calculated from eq 1 after Mason and Gandhi (1980). eApproximate temperature a t which the indicated product yield is observed. Yield still increasing as temperature increases. ‘Insufficient data to determine. fHigh temperature yields of molecular hydrogen were estimated to be 0.2 wt % and were ignored in this analysis.
n
8%
I
1
I
I
1
7.0 HzO
n
1 K
0.25. 420
I
600
800 1000 1200 1400 PEAK T E M P E R A T U R E . K
Figure 5. Effect of peak temperature on yield of water from pyrolysis of sweet gum hardwood powder. [Points: experimental data, curves: trendlines; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; powder size range = 45-88 pm.] 0
2
L
1.50
I
I
I
I
I
1
I
6 0 0 800 1000 1200 1400 PEAK TEMPERATURE, K
Figure 7. Effect of peak temperature on yield of ethane from pyrolysis of sweet gum hardwood powder. [Points: Experimental data; curves: trendlines; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; powder size range = 45-88 pm.] n 0
0 0.5-
C3Hg ,
4
p
/
b
I
0.3
2
5 w> 600
800 loo0 1200 1400 PEAK TEMPERATURE , K
0.2
o.l/
I
I
j
o 600
800 1000 1200 1400 PEAK TEMPERATURE, K
Figure 6. Effect of P e d temperature on yield of ethylene from pyrolysis of sweet gum hardwood powder. [Points: experimental data; curves: trendlines; pressure 5 psig (helium); nominal heating rate = loo0 K/s; powder size range = 45-88 pm.]
Figure 8. Effect of peak temperature on yield of propylene from pyrolysis of sweet gum w o o d powder. [points: experimental data; curves: trendlines; pressure = 5 psig (helium); nominal heating rate = 1000 K/s; powder size range = 45-88 pm.]
Figure 1shows the yields of char, tar, and gas (including water). Under the present conditions decomposition of the wood is first observed at about 600 K. Sample weight loss increases with temperature until 93% of the wood is converted to volatile material at 950 K. Above this temper-
ature, the char yield remains constant at 7 w t % Most of the sample weight loss occurs between 700 and 900 K. Tar and gas are initially evolved at the same rate (at 600 K), but tar production becomes much greater as the peak temperature is increased above 700 K. As temperature
.
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 839
n 2.51
CH2O 0
o o o o
s
0
0
t 600
800
PEAK
1000
1200
1400
TEMPERATURE, K
Figure 9. Effect of peak temperature on yields of formaldehyde from pyrolysis of sweet gum hardwood powder. [Points: experimental data; curves: trendlines; pressure = 5 psig (helium); nominal heating rate = lo00 K/s; powder size range = 45-88 pm.]
increases, tar yield goes through a maximum of about 55 wt % at 850-950 K and attains an essentially asymptotic yield of 46 wt % at 1300 K. The decrease in tar yield at
temperatures above 950 K is believed to arise from its secondary cracking to light volatiles. Secondary cracking of biomass tars to form light volatiles was also observed in rapid pyrolysis of cellulose in this apparatus (Haljaligol et al., 1982). Figure 1shows that the decline in tar yield from the sweet gum falls off dramatically, but (based on CH, and C2H4high-temperature yield data, see below) not completely, by 1300 K. One interpretation is that primary pyrolysis of sweet gum produced two broad classes of tars, one distinctly thermally labile under the present conditions, the other essentially unreactive (Nunn et al., 1985). As temperature increases above 950 K, most of the reactive tar is converted to either light volatiles or additional unreactive tar by 1300 K. Formation of pyrolytic tars of significantly different thermal stability is implied by other biomass pyrolysis studies (Hajaligol et al., 1982; Wenzl, 1970; Stamm and Harris, 1953),and similar behavior has been reported more recently for coal (Serio, 1984; Serio et al., 1983,1984). An alternative view is that the observed behavior reflects an apparatus-specific effect in which a certain fraction of the tar is formed and escapes the neighborhood of the screen before the temperature of that region becomes high enough to cause secondary reactions. The present experiments and those of Hajaligol et al. (1982) were performed nonisothermally and did not isolate primary and secondary reactions for independent study as did those of Serio et al. on coal tars (Serio, 19M Serio et al., 1983,1984). Such studies would be useful in distinguishing between the above and other interpretations of wood thermal reactivity. Figure 2 shows the yield of CO as a function of peak temperature. This product is first observed at about 750-800 K and its yield increases monotonically with temperature to an asymptote of 17 wt % near 1200 K. Since significant additional amounts of CO are produced at temperatures above 950 K, where weight loss (Le., 100% minus the char yield, see Figure l),and hence primary decomposition of the sample has essentially ceased, it is concluded that CO is also a major product of secondary cracking of the tar at elevated temperatures. The incremental CO yield between 950 and 1200 K (about 8 wt %) accounts for most of the 9 wt % decrease in tar yield between these two temperatures. On a weight basis, CO dominates the gaseous produds at temperatures above 900 K. The effect of peak temperature on the yield of CHI is shown in Figure 3. Its production starts around 800 K and
increases rapidly with temperature to a yield of about 1.3 wt % at 950 K. Its yield continues to increase with temperature above this point but more slowly than at lower temperatures. Given the essentially constant char yield above this temperature, these observations are consistent with the view that, in analogy to CO evolution, secondary tar cracking reactions contribute to the high-temperature CHI yield. The absolute magnitude of this inferred secondary reaction contribution is about 1wt % of the parent wood. Unlike CO, CHI yield continues to increase with increasing temperature above 1200 K, to a value of 2.3 wt % at 1520 K. Comparison of Figure 3 with Figure 1shows that the incremental CHI production between 1200 and 1520 K (0.5 w t % of wood) falls within the uncertainty in the tar yield curve, implying that secondary tar cracking continues up to the highest temperature studied. Figure 4 illustrates the yield vs. peak temperature behavior of COz. Measurable quantities of this product are found at temperatures as low as 600 K. Its yield increases with temperature, leveling off at 6 wt % near 950 K. This behavior suggest that COzis derived mainly from the direct degradation of the wood and that the secondary cracking reactions contributing to CO and CHI yields do not contribute significantly to COPproduction under the present conditions. The yields of chemical (nonmoisture) water are shown in Figure 5. The data exhibit somewhat more scatter than several lighter components. This is believed to be due to severe tailing of the water peak during the GC analysis (Cosway, 1981). Even with this scatter the broad trends in the data are readily discerned. Water is evolved immediately after decomposition of the wood begins at approximately 600 K. This observation is consistent with the postulate that the major pathways for water formation from lignocellulosicsare dehydration and depolymerization reactions, whch can occur at low temperatures (Hajaligol, 1980). The water yield increases monotonically with temperature and plateaus at 5.0 wt % at 900 K. The yields of several light hydrocarbon gases and of formaldehyde are shown in Figures 6-9. Figure 6 shows the yield of ethylene (CzH4) as a function of peak temperature. Ethylene evolution is first observed at 800 K and its yield increases steadily with temperature. At around 950 K there is a change in the slope of the C2H4 yield curve, and its yield increases more slowly with further temperature increases. The maximum yield measured for ethylene was 1.4 wt % at 1520 K, which may not be an asymptotic value. This behavior indicates that the incremental production of C2H4between 950 and 1520 K (approximately 0.6 wt % of wood) arises from secondary cracking of tar as discussed above for CH4 The evolution of ethane (C2H6) is illustrated in Figure 7. Measurable C2H6 yields are seen at 800 K, with production increasing with temperature to a plateau of 0.17 wt % at 950 K. Approximately 60% of this yield occurs above 900 K where significant secondary cracking of the tar is occurring (Figure l),suggesting that this pathway is an important reaction channel for C2H6 production during rapid pyrolysis. As shown in Figure 8, propylene (C,H,J is evolved in a manner that closely parallels the yield behavior of ethane. Propylene production begins at a peak temperature near 800 K, increases strongly with temperature between 850 and 900 K,and levels off at its ultimate yield of 0.42 wt % near 950 K. The propylene data exhibit some scatter which is probably the consequence of occasional water interference on the GC. Secondary tar cracking is also believed to play a significant role in C3H6 evolution since
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
around 25% of the C3H, is evolved above 900 K where net tar production reflects secondary cracking. The yield data for many of the light oxygenated compounds observed in this work, including methanol, acetaldehyde, ethanol (+ butene), acetone + furan, and acetic acid, exhibited such a high degree of scatter that it is difficult to reliably discern trends in their yield vs. temperature behavior. In addition to the uncertainty created by the tailing of the water peak in the GC analysis, minute amounts of residual methanol/acetone solvent used for tar recovery (Nunn, 1981) can cause extremely large errors in the measured yields of methanol and acetone/furan. For example, 0.001 mL of residual acetone would contribute an uncertainty of 100% to the observed pyrolysis yield of this product. Fortunately, some of the high-temperature runs have a sufficient degree of reliability to allow apparent ultimate yields of these products to be estimated with confidence. These findings are discussed below and summarized in Table 11. The formaldehyde (HCHO) data generally exhibited less scatter than those of the other light oxygenates listed above. Figure 9 shows that this product is observed at about 600 K and its yield increases monotonically with temperature to a reasonably well-defined asymptote of 2.0 w t % above 900 K. The appearance of about 50% of the asymptotic yields for temperatures below those expected to cause significant tar cracking (Le., by 790 K; compare Figure 1)suggests that primary decomposition of the wood is at least partly responsible for its production. Material, Elemental, and Energy Balances. Elemental analyses of the parent wood and of product tars and chars obtained a t different peak temperatures are presented in Table I. The char shows increasing carbon and decreasing oxygen content with increasing temperature, suggesting that oxygen functionalities are more labile in the parent wood. The maximization of tar oxygen content with increasing temperature suggests that oxygen-containing compounds are significant constituents of tar formed at intermediate temperatures and that some of these compounds are preferentially removed by secondary cracking at higher temperatures. Further, in light of the above discussion on evolution of the carbon oxides under the present conditions, it seems likely that much of this tar oxygen ends up as CO. Table I1 presents elemental, total mass, and energy balances for pyrolysis of this sweet gum. Since some of the data for the light liquids are highly scattered, this analyses was performed using estimated ultimate yields rather than the data from a specific experimental run. The table includes estimates of the peak temperature 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 compounds to the ultimate product spectrum is small (about 2.2 wt %). Also, since no elemental analysis was available for high-temperature char, this product was assumed to be 100% carbon. Heats of combustion for the individual gas products were taken from CRC (1976) and the values for wood, char, and tar were calculated using a 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 (01, respectively, are the carbon, hy-
drogen, and oxygen contents in weight percent. The balances for total mass, carbon, and hydrogen are very good. The oxygen balance is somewhat low, perhaps due to the assumptions that the char and miscellaneous oxygenated compounds contain no oxygen, or as a result of errors in the elemental analysis of char, tar and wood. The energy balance in Table I1 shows that the asymptotic char make accounts for only 12.1% of the wood energy content, while the high-temperature tar and product gases account for over 62% and 30%, respectively. Carbon monoxide and methane together contain more than half of the gaseous heating value. Although on a weight basis the asymptotic CO yield is over seven times greater than that of methane, methane accounts for almost as much of the energy of the wood (6.5%) as does CO (8.7%). The total energy content of the individual products exceeds that of the parent wood by less than 5% under the above conditions. This is within the uncertainty of the present analysis, and the high-temperature, thermal decomposition of sweet gum hardwood would thus appear to be 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; and Lewellen et d.,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
dVi -= dt
(Vi* - Vi)koi exp
where ka and Ei are 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 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 koi,Ei, and Vi* (i.e., those that minimize the sum of squared errors between calculated and observed yields) are derived by comparing integral product yields Vi(T?,measured in nonisothermal experiments to different peak temperatures T’,and of overall duration t’, with corresponding values calculated using measured laboratory time-temperature histories, a nonlinear least-squares regression code, and the integrated form of eq 2
[
]
Vi* - Vi(T’)
-%)
= -Jt’koi exp( dt (3) vi* In 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
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
Table 111. Kinetic Parameters for Sweet Gum Pyrolysis Ei,kcal/ std error of product E-mol log k,, s-l V*, wt % est: w t % weight loss 16.5 4.53 92.97 7.66 2.88 41.01 2.54 total gases 11.8 2.15 3.36 17.05 co 14.6 16.6 3.79 1.91 0.21 CHI 14.3 3.77 5.97 0.51 CO2 CZHl 19.2 4.41 1.17 0.12 5.87 0.17 0.02 C2H6 23.7 3.35 5.14 0.65 H20 11.5 3.51 1.99 0.27 HCHO 12.9 3.26 7.13 0.74 H20 + HCHO 11.5 11.20 0.41 0.06 C3H6 42.8 0.34 CHBCHO 21.3 5.80 1.40 "Defined as EM(VJ,,,,del number of data points.
VJ,,,,#/(n
- 3)]*/2,where n is the
2>
i
%
L
o 600 PEAK
BOO 1000 1200
841
1400
TEMPERATURE, K
Figure 12. Experimental data (0) and model-generated curve for carbon monoxide production from pyrolysis of sweet gum hardwood powder. [Pressure = 5 psig (helium);nominal heating rate = 1000 K/s; powder size range = 45-88 pm.]
5 IL
0
2.5
PEAK T E M P E R A T U R E , K
Figure 10. Experimental data (0) and model-generated curve for total weight loss from pyrolysis of sweet gum hardwood powder. [Preasure = 5 psig (helium);nominal heating rat = lo00 K/s; powder size range = 45-88 pm.]
O
:w 4 w
7
I
I
10
o/ 600 800 °1000 1200 1 4 0 0 L 600 800 1000 1200 1 4 0 0 PEAK T E M P E R A T U R E , K
Figure 11. Experimental data (0) and model-generated curve for total gas production from pyrolysis of sweet gum hardwood powder. [Pressure = 5 psig (helium); nominal heating rate = lo00 K/s; powder size range = 45-88 pm.]
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 weight loss, and for yields of gas, CO, and CHI, are compared to the laboratory data in Figures 10-13, 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 ob-
600 800 1 0 0 0 1 2 0 0 1 4 0 0 PEAK T E M P E R A T U R E , K
Figure 13. Experimental data (0) and model-generated curve for methane production from pyrolysis of sweet gum hardwood powder. [Pressure = 5 psig (helium);nominal heating rate = 1000 K/s; powder size range = 45-88 pm.]
served CHI yields (Figure 13) 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. Implementation of the latter, 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 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. The present yield vs. temperature data indicate an important role for multiple chemical pathways in the production of CO, CH4,C2H4,and other light gases, and in tar evolution at higher temperatures. Further, except for C3H6,the activation energies and accompanying preexponential factors in Table I11 are much 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 (Juntgen and Van Heek, 1970; Howard, 1981b).
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
Table IV. Single-Reaction, First-order Kinetic Parameters for Total Weight Loss in the Pyrolysis of Several Woods temp E, ultimate range studied, kcaI/gyield, V*, investigators reactor type wood type sample dimensions, pm K mol log ko, s--l wt % Stamm (1956) bath of molten Sitka spruce 1588 X 25400 X 149200 440-573 29.8 7.45 12 metal veneer Roberta and Clough tube furance beech cylinder 20000-21 400 0.d. X 626-778 15 3.18 72 (1963) 130 000-145 000 Thurner and Mann tube furnace Missouri oak sawdust, surface mean size 615 573-665 25.4 5.87 70 (1981) Nunn et al. (this screen heater sweet gum powder 45-88 573-1373 16.5 4.53 93 work) TEMPERATURE, K 1400
1MO
800 700
600
10
-401
; \i
- 7 0 1
-BOL-L-L.-L, 08
10
I 12
RECIPROCAL
!
1 14
I
1 16
ABSOLUTE
18
20
2 2
T E M P E R A T U R E r1000.K“
Figure 14. Kinetic data for total weight loss from the pyrolysis of several woods. Samples types and experimental details are given in Table IV.
Table IV and Figure 14 compare the kinetic parameters for total weight loss from sweet gum hardwood from this work, with those for other woods presented by Thurner and Mann (1981), Roberta and Clough (19631, and S t a ” (1956). In the regions of overlapping temperature, the present findings predict higher rates and extents of overall devolatilization. Some of these disparities may be from differences in wood type. Further, the data of Roberts and Clough (1963) and Stamm (1956) were obtained from samples of larger dimensions and probably reflect a significant contribution from intraparticle char-forming secondary reactions which would reduce the overall rates and extents of weight loss. On the other hand, Roberts (1970) has argued that larger sample dimensions enhance intraaarnple autocatalytic reactions which increase overall rates of weight loss. Further research on the role of secondary reactions in wood pyrolysis is clearly needed. Figure 14 also implies that: (a) extrapolating kinetic data outside the range of temperature over which they were obtained may cause serious errors, and (b) derivation of reliable kinetic parameters from wood pyrolysis experiments and successful utilization of those parameters in reactor engineering calculations requires that secondary reactions be accounted for. Conclusions The following conclusions on the rapid pyrolysis behavior of sweet gum hardwood can be drawn from this study. (1) The captive sample electrical screen heater reactor is a reliable tool for determining product yields, compo-
sitions, and global evolution kinetics in the rapid pyrolysis of wood under the present conditions. Good material and C and H elemental balances have been achieved and product energy values are also readily estimated. (2) High conversions of the wood to volatile products can be obtained in very short substrate residence times (51 s) with an asymptotic weight loss of 93% occurring above 1100 K. (3) Tar is the major pyrolysis product at temperatures above 800 K. It exhibits a maximum yield of 55 wt % at 900 K where it accounts for about 61% of the heating value of the pyrolysis products. Although its high-temperature yield has declined to 46 wt ?& at 1300 K, this represents 62% of the energy content of the parent wood because this tar is richer in carbon and hydrogen and lower in oxygen than that collected from the 900 K run. (4) Secondary cracking of this tar contributes significantly to the yields of CO, CHI, CzH4 and most of the other light gaseous products at temperatures above 900-950 K. (5) Total gas production (including chemical, i.e., nonmoisture water) increases monotonically with increasing temperature to an ultimate yield of 40 wt % above 1300 K, which represents 30% of the wood energy content. (6) On a weight basis, CO is the dominant gaseous product above 850 K, reaching an ultimate yield of 17 wt % by 1300 K. Below 850 K, COz and chemical water dominate the gas make, and attain close to asymptotic yields of 5.7 and 5.1 w t % at 1000 and 900 K, respectively. The CHI yield (2.3 wt %) at 1520 K represents almost as large a proportion of the parent wood energy (6.5%)as does the 8.7% accounted for by the asymptotic CO yield of 17 wt %. (7) Although their yield increases with temperature, light hydrocarbon gases (CHI, CZH4, C2H6,C3H6)are relatively minor products with their total yield at 1300 K remaining below 5 wt %. (8) Light oxygenated liquids, such as formaldehyde, methanol, and acetaldehyde, cumulatively account for approximately 9 wt % of the wood at peak temperatures above 1300 K. (9) The global rates of evolution of most major products except tar were well fitted by a single reaction first-order decomposition kinetic model. However, the detailed mechanisms for products generation are in general more complex than single-step pathways. This simple model should therefore be viewed only as a convenient tool for correlating data and comparing results from different bicimass materials under similar reaction conditions. (10) Rates of total weight loss were higher than those reported by other investigators, probably because of reduced contributions from apparatus-specific secondary char-forming reactions in the present equipment, and differences in wood compositions. (11) Much remains to be learned about the fundamental chemical and physical processes governing the pyrolytic
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
decomposition of biomass. However, the present results show that for heating rates of 1000 K/s, primary decomposition of the sample to tar is a major pathway in the rapid pyrolysis of sweet gum below 900 K. Above 900 K, secondary cracking of tar contributes significantly to the product spectrum. Figure 1shows that char formation is also an important pathway a t temperatures up to 900 K. The present data do not allow one to determine what fraction of this char is formed by recondensation of liquid intermediates such as tar,generated by early bond breaking of the substrate. However, previous studies on filter paper cellulose showed that major increases in tar yields were obtained at lower temperatures (5700-800 K) by using lower heating rates or longer holding times (Hajaligol et al., 1982), i.e., at conditions disfavorable to secondary condensation reactions. This observation suggests that char from wood may also be a secondary product, especially since compared to cellulose tars, wood tar contains more aromatics which would be expected to have stronger coke forming tendencies.
Implications for Industrial Practice The present findings identify potential opportunities for manipulating operating conditions in larger scale reactors to commercial advantage. A major issue is control or redirection of secondary, i.e., post-pyrolysis, reactions of volatiles, especially tars. For example, repolymerization of transportable pyrolysis products to heavy liquids and char would be expected on the surface of, and particularly within, the reacting substrate. Reducing the minimum dimension of the sample would disfavor this pathway. The present results show that given sufficiently small particle diameters, large yields of tar liquids can be achieved by minimizing extra-particle secondary reactions, Le., secondary cracking in the particle boundary layer and carrier gas. This end could in turn be achieved by independently controlling substrate and volatiles temperature-time histories, with reduced volatiles residence times and temperatures favoring high liquids yields and the converse resulting in augmented production of light gases such as
co.
It is also reasonable to expect that a desire to avoid feedstock comminution costs will cause many biomass thermochemical conversion processes to be designed for optimal particle dimensions substantially larger than those studied here. In the absence of mechanically assisted removal of tars and other volatiles from the reacting sample (so-called ablative pyrolysis, see Diebold and Scahill, 1984; Lede et al., 1983; Villermaux et al., 1983), larger particles would, as noted above, offer increased opportunity for secondary, char-forming reactions. Thus the low char yields (high total weight loss) above 1000 K observed in the present investigation may represent an upper limit on sample conversion to potentially valuable gaseous and liquid products, that may not be attainable for larger particles in non-ablative reactors. The present observations can thus be viewed as identifying reasonable expectations for upper bounds on total volatiles yields in larger scale equipment. The present results also confirm the need to obtain quantitative understanding of the separate contributions of primary decomposition and of volatiles secondary reactions to the overall thermal conversion behavior of biomass. Both intraparticle and extraparticle phenomena are of interest. Such information would be essential for reliably predicting the thermal reaction behavior of biomass in practical scale reactors where secondary char forming
843
and volatiles cracking reactions cannot be totally arrested.
Acknowledgment We gratefully acknowledge the financial support for this work provided by the U.S. Department of Energy under Grant No. DEFG02-79ETOOO84 and by the Edith C. Blum Foundation of New York, NY. The DOE grant was administered by the Solar Energy Research Institute, and Drs. A. Kotch and T. A. Milne have served as technical project officers. Samples of sweet gum hardwood, milled wood lignin, and other wood constituents were specially prepared for this work by Professor H.-M. Chang and his colleagues in the Department of Wood and Paper Science at North Carolina State University, who are gratefully acknowledged. We also acknowledge important contributions by P. Bhadha, R. Caron, J. Curme, H. D. Franklin, J. Ghosh, M. R. Hajaligol, P. Houghton, J. Rau, and S. G. Tdiaferro. The Robert C. Wheeler Foundation, Palo Alto, CA, underwrote publication costs.
Literature Cited Allan, 0. 0.; Matllla, T. I n ”Llgnlns Occurrence, Formation, Structure, and Reactions”: Sarkanen, K. V.; Ludwlg, C. H., Ed.; Wiley-Interscience; New York, 1971. Antal. M. J. Chapter I 1 In VoI. 111, Part C of “Energy from Biologlcel Processes”; Offlce of Technology Assessment, United States Congress, Report No. PB-81-134793, Sept 1980. Cosway, R. Q. M.S. Thesls, Department of Chemical Engineering, Massachusetts Instltute of Technology, Cambridge, MA, 1981. “Handbook of Chemistry and Physics”, 57th Ed.; Weast, R. C., Ed., CRC Press: Clevelanc!: OH, 1976. Dlebold, J. P., Ed., Proceedings Speclallsts Workshop on Fast Pyrolysis of Blomass”; Report No. SERI/CP-622-1096, The Solar Energy Research Institute: Golden, Co, 1981. Dleboid, J. P.; Scahill, J. W. I n “Proceedings, Internatlonal Conference on Fundamentals of Thermochemical Biomass Conversion”, Overend, R. P., Mllne. T. A.; Mudge, L. K., Ed.: Applled Sclence Publishers: London, 1984. Franklin, H. D.; Peters, W. A.; Cariello, F.; Howard, J. B. I d . €ng. Chem. Rocess Des. Dev. 1881, 20, 670. Qhosh, J.; Howard, J. B.; Longwell, J. P.; Peters, W. A. to be submitted for publlcetion In I d . Eng. Chem. process Des. D e v . , 1985. Hajaligol, M. R. Ph.D. Thesls, Department of Chemical Englneerlng, Massachusetts Institute of Technology, Cambridge, MA, 1980. Hajallgol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. I d . Eng. Chem. Process Des. Dev. 1882, 21. 457. Howard, J. B. I n “Chemistry of Coal Utllkation, Second Supplementary Volume”, Ellbtt, M. A., Ed.; Wlley: New York. 198la; Chapter 12; 198lb; p 737. Juntgen, H.; Van Heek. K. H. Fortschr. Chem. Fcfsch. 1970. 13, 601-699. Klein, M.T. Sc.D. Thesls, Department of Chemlcal Engineering. Massachusetts Instltute of Technology, Cambridge, MA, 1981. Lede, J.; Panagopoulos, J.; Vlllermaux. J. Repr.. Div. Fuel Chem., Am. Chem. Soc. 1983, 28(5),383. Lewellen, P. C.; Peters, W. A.; Howard, J. 8. “Proceedings, Slxteenth S y m poslum (Internatlonal) on Combustion”; The Combustlon Institute, Pittsburgh, 1977; p 1471. Mason, D. M.; Qandhl, K. Repr., Dlv. FuelChem., Am. Chem. Soc., 1880, 25(3), 235. Mline, T. A. Chapter 5 In “Principles of Gasification”, Vol. I 1 of “A Survey of Blomass Qasificatlon,” Reed, T. B., Ed.; Report No. SERI/TR-33-239, The Solar Energy Research Institute, Golden, CO, 1979. Molton, P. M.; Demmltt, T. F. “Reaction Mechanisms in Cellulose Pyrolysis A Literature Review“; Report No. BNWL-2297, Batteile Paclflc Northwest Laboratories, Richland, WA, 1977. Nunn, T. R. M.S. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1981. Nunn, T. R.; Howard, J. 8.; Longwell. J. P., Peters, W. A. Ind. Eng. Chem. Process Des. Dev. 1885, followlng paper in this issue. Peters, W. A. “Literature Incentives for the Production of Clean Fuels and Chemicals from Blomass by Thermal Processlng”; In-house Literature R e view, Energy Laboratory, Massachusetts Institute of Technology, Cambridge. MA. 1978. Raines, D. Huffman Laboratories, Inc., Wheatridge, CO, personal communication, 1981. Roberts, A. F. Combust. Flame 1870, 14. 261. Roberts, A. F.; Clough, 0. ”Proceedings, Nlnth Symposlum (Internatlonal) on Combustion”; Academic Press: New York, 1963 p 158. Serlo, M. A. Ph.D. Thesls, Department of Chemical Englneerlng. Massachusetts Institute of Technology, Cambridge, MA, 1984. Serb, M. A.; Peters, W. A.; Sawada, K.; Howard, J. 6. “Proceedings. International Conference on Coal Science, Pittsburgh, Aug 15-19, 1983”; International Energy Agency, 1983; p 533. Serb M. A.; Peters, W. A.; Sawada. K.; Howard, J. B. Repr., Div. Fuel Chem., Am. Chem. SOC.1984, 29(2), 65. Stamm, A. J. I d . Eng. Chem. 1956, 4 8 , 413. Stamm, A. J.; Harrls. E. E. “Chemical Processing of Wood”; Chemlcal Publishlng Co.: New York, 1953.
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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
