I n d . Eng. Chem. Res 1988,27, 2169-2174
Oseiran and Jean Breton is also acknowledged. Reaistrv No. HC1. 7647-01-0: HNO,. 7697-37-2: CaC1. Ca(HiP04j2, 108373-6818; Ca(N03)2.Ca(HiP04)2,97102-02-8; CaH3P04, 7757-93-9.
Literature Cited Anonymous, "The Florida Phosphate Slimes Problem. A Review and a Bibliography". Inf. Circ.-U. S., Bur. Mines 1975, 8668. Anonymous International Solution Mining- Symposium; AIME: . Littleton, CO, 1981. Anonvmous "The Florida PhosDhate Industrv's Technoloeical and Enkronmental problems. Review". I i f . Circ.--U.-S., Bur. Mines 1983, 8914. Awadalla, F. T.; Habashi, F. "The Removal of Radium During the Production of Nitrophosphate Fertilizer". Radiochim. Acta 1985, 38, 207-210. Balazik, R. F. "Costa and Effects of Environmental Protection Controls Regulating US Phosphate Rock Mining". Inf. Circ.-U. s., Bur. Mines 1983,8932. Dahl, J. L. "Solution Mining Bibliography". In Salts &Brines '85, Schlitt, W. J., Ed.; AIME: Littleton, CO, 1985; pp 169-204. Erlenmeyer, E. "Uber Bildung und Zusammensetzung des so-genanteen sauren phosphorsauren Kalk". Heidelberg, 1857; Quoted in A Comprehensive Treaties on Inorganic and Theoretical Chemistry; Mellor, J. W., Ed.; Longmans Green: London, 1923; Vol. 3, p 902. Fox, E. J.; Clark, K. G. "Monocalcium Chlorophosphate". Ind. Eng. Chem. 1938, 30(6), 701-703. Fox, E. J.; Clark, K. G. "The Chlorophosphate Process for Dicalcium Phosphate". Ind. Eng. Chem. 1943,35(12), 1264-1268. Frazier, A. W.; Lehr, J. R. "A New Calcium Nitrate Phosphate". Agric. Food Chem. 1968,16, 388-390. Gmelin's Handbuch der Anorganischem Chemie; Verlag Chemie: Weinheim, 1961; Vol. 28, "Calcium", Part B3. Habashi, F. "Hydrometallurgy". In Principles of Extractive Metallurgy; Gordon & Breach: New York, 1969 (reprinted 1980); VOl. 2. Habashi, F.; Awadalla, F. T.; Yao, Xian-Bao "The Hydrochloric Acid Route to Phosphate Rock". J. Chem. Technol. Biotechnol. 1987, 37, 371-383. Hignett, T. P., Ed. Fertilizer Manual; International Fertilizer Development Center: Muscle Shoals, AL, 1985.
2169
Hiskey, J. B., Ed. Gold and Silver Heap and Dump Leaching Practice; AIME: Littleton, CO, 1984. Hughes, A. E.; Cameron, F. K. "Action of Sulfur Dioxide on Phosphates of Calcium". Ind. Eng. Chem. 1931, 23(11), 1262-1271. Lawver, J. E.; McClintock, W. 0.; Snow, R. E. "Beneficiation of Phosphate Rock. A State of the Art Review". Miner. Sci. Eng. 1978, 10(4), 278-294. Lawver, J. E.; Bernardi, J. P.; McKereghan, M. F.; Raulerson, J. D.; Lynch, D.; Hearon, R. S. "New Techniques in Beneficiationof the Florida Phosphates of the Future". Miner. Metall. Process. 1984, 1 ( 2 ) , 89-106. Raulerson, J. D.; Williams, J. M. "Evaluation of Florida Phosphate Matrix Transportation from Mine to Plant". Min. Eng. 1983, 35(10), 1413-1418. Scheiner, B. J.; Smelley, A. G.; Brooks, D. R. "Large Scale Dewatering of Phosphatic Clay Waste from Central Florida". Rep. Invest.-U. S., Bur. Mines 1982, 8611. Schlitt, W. J., Ed. In-situ Uranium Mining and Ground Water Restoration; AIME: Littleton, CO, 1979. Schlitt, W. J., Ed. Salt and Brine '85. Proceedings Symposium Solution Mining of Salts and Brines; AIME: Littleton, CO, 1985. Schlitt, W. J.; Hiskey, J. B., Eds. Interfacing Technologies in Solution Mining; AIME: Littleton, CO, 1981. Taperova, A. A.; Shul'gina, M. N. "PhysicochemicalAnalysis in the Field of the Hydrochloric Acid Treatment of Phosphates. I. The System Ca0-P205-HC1-H20". J. Appl. Chem. U S S R (Engl. Transl.) 1946,19,1350-1357; Chem. Abstr. l946,41,7219f. Vol'fkovich, S. I.; Loginova, A. A. "Process of Working up Apatite with HCl to Produce Phosphate Fertilizer, Rare Earths, and Fluorine Salts". Dokl. Akad. Nuuk S S S R 1944, 44, 168-171; Chem. Abstr. 1944,39, 1260. Walter-Levy, L.; Maarten de Wolff, P.; Vincent, J. P. "Calcium Chlorophosphates". C. R. Acad. Sci. 1955,240, 308-310; Chem. Abstr. i95< 49, 6763h. White. J. C.: Fereus. A. J.: Goff. T. N. "PhosDhoric Acid bv Direct Sulfuric Acid bigestion of Florida Land-Pebble Matrix"". Rep. Invest.-US., Bur. Mines 1975, 8086. Wilson, H. N. "The Determination of Phosphate in the Presence of Soluble Silicates. Application to the Analysis of Basic Slag and Fertilizers". Analyst 1954, 75, 535-546. Received f o r review February 23, 1988 Revised manuscript received June 3, 1988 Accepted June 30, 1988
Moderately High Temperature Pyrolysis of Lignocellulose under Vacuum Conditions Mbanefo M. Ekwenchi,* Benedict E. Araka, and Kieran I. Ekpenyong Department o f Chemistry, University of Jos, Jos, Nigeria
Pyrolysis of lignocellular material (prepared from elephant grass, panicum maxima) was performed in the temperature range 200-300 "C under constant and variable loading conditions a t constant reaction time of 2 h. Gases, volatiles, nonvolatiles, and tar are observable products of the pyrolysis. Higher total yields of hydrocarbons evolved were achieved both at higher temperatures and higher loading conditions. Butane is found to be the most dominant of the C1-C6 hydrocarbons, and c15-c35 alkanes (n-alkanes) are identified in the saturated hydrocarbons of the nonvolatiles. T h e C15-C35 n-alkenes are identified in an appreciable quantity. The rates of evolution of C1-Cs and saturated hydrocarbons follow first-order kinetics. The activation energies for the formation of C2-C6 and saturated hydrocarbons fall in a very narrow range, 6.4-10.2 kcal mol-l, while that of CH4 is noticeably higher, 28.7 kcal mol-'. We have investigated under vacuum conditions, the pyrolysis of lignocellulose prepared from elephant grass, panicum maxima, which is one of the most abundant and widely grown grasses in Nigeria. The lignocellular material is found to be thermally unstable even at moderately high temperatures, yielding reasonable amounts of decompo-
* To whom correspondence should be addressed. 0888-588518812627-2169$01.50/0
sition products around temperatures of 200 "C and above (Roy and Chronet, 1981). Some experiments on the production of oil from cellular materials have been reported (Appel, 1977; Cavelier and Chronet, 1977; Appel et al., 1971; Boocock et al., 1979,1980; Molton et al., 1981; Elliot, 1980; Eager et al., 1981, 1983). These studies involved the high temperature-high pressure reaction of carbon monoxide and/or a suitable catalyst. 0 1988 American Chemical Society
2170 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table I. Product Yields for the Pyrolysis of Lignocellulose as a Function of Sample Weight yields," mg sample wt, mg
CH4
CzH4
cas
CSH8
n-C*Hlo
CSHlO
n-CJ-42
10 15 25
0.189 0.480 0.819
0.0436 0.121 0.195
0.0566 0.157 0.254
0.0499 0.107 0.200
47.9 70.4 124.0
12.7 28.0 55.6
11.9 26.2 52.1
a
i-C2H12 12.3 21.2 54.1
n-C6H14
13.6 22.7 37.8
Pyrolysis temperature 200 "C; pyrolysis time, 2 h.
However, quantitative studies of the kinetic parameters related to the chemical transformations of the lignocellular materials by a pyrolytic process under vacuum conditions have not been performed. Because little work has been done on the pyrolysis of agricultural wastes (Waterman, 1976; Antal, 1978; Kuestar, 1979; Diebold, 1980),which represent the potential sources of the lignocellular materials, the kinetic data are sparce, the products formed and the product distribution have not been well characterized, so we decided to investigate the thermal behavior of lignocellulose in order to gain an insight into the nature of products formed under specified conditions as well as the kinetic parameters involved in the chemical transformations. Such insights are expected to lead to a better understanding of the nature of the gases released to the atmosphere during forest or bush burning for agricultural purposes, which poses environmental pollution problems. Also, it is envisaged that the relative importance of these products as fuels and chemical feedstocks of the future will be evaluated. We now report the results on the kinetics of light hydrocarbon gas evolution and nonvolatiles-saturated hydrocarbons formed in the temperature range 200-300 "C and on the gross composition of the total yields of the pyrolytic products in the same temperature range.
Experimental Section Preparation of Lignocellulose. Lignocellulose was prepared from an elephant grass according to the method previously described (Crawford, 1978). The grass was collected from the locality of Jos, Nigeria, dried in an oven at 105 "C, and ground to a mesh size of less than 250 Fm. The material was then extracted firstly by reflux with hot water, secondly with a benzene and ethanol mixture (l:l, v/v), and finally with an ethanol and hot water mixture ( l : l , v/v), each for a duration of 5 h. The resultant material was dried in an oven at 105 "C for 24 h. The resultant lignocellulose was subsequently Soxhlet extracted with benzene for 72 h to yield the "purified lignocellulose". Pyrolysis of Lignocellulose. For bulk analysis of the product, a special vacuum apparatus was constructed for the study. About 1-2 g of lignocellulose was placed in a suitable pyrex reactor bulb which was attached to a vacuum system. The bulk of the air in the bulb was removed by rapid pumping (evacuation) for 20 min. The pressure in the reactor bulb dropped to about 0.1 Torr. Pumping was then discontinued, and the bulb was sealed off and heated to the desired temperature in a temperature-controlled oven for the required length of time. The bulb was then cooled in an ice bath, and the seal was then broken to release the gases and the volatile reaction products. For the detailed analysis of the gaseous products, the apparatus was constructed specially, providing for the injection of whole gaseous reaction products into the gas-liquid chromatograph. This consisted of a reaction tube (4 in. x 1/4 in.) which after pyrolysis could be inserted in a high-temperature, stainless steel tube cracker. Analysis of Gaseous Products. For the identification of individual components, c& hydrocarbons, a gas-liquid chromatograph, a Pye Unicam, Model 104, fitted with
a tube cracker precolumn (6 in. X 1 in.) followed by Porapak Q (6 f t X '/8 in.) in a stainless steel column was used. The identity of each component was confirmed by gasliquid chromatography retention time comparisons with authentic samples. For quantitative determinations, the gas-liquid chromatograph was calibrated for each component by standard techniques. The column was temperature programmed between 50 and 180 "C, rising at 8 "C/min after an initial 5-min period. The carrier gas was helium flowing at 28 cm3/min, and effluent was monitored by using a flame ionization detector. Analysis of Heavier Hydrocarbons and Other Compounds. After pyrolysis, the nonvolatile material was quantitatively transferred to the Soxhlet extractor. The heavy products were Soxhlet extracted with benzene for 72 h, which yielded "benzene extract". The weight of the unpyrolyzed material was determined after extraction while the benzene extract was evaporated to dryness and weighed. The following steps were involved in the analysis of the benzene extract. (i) Precipitation of the Tar Material. The benzene extract was dissolved in 1 cm3 of benzene, and 40 cm3 of petroleum spirit was quickly added to the benzene solution, stirred by a magnetic stirrer and kept in a freezer for about 1 2 h. The petroleum spirit soluble extract was decanted quantitatively, evaporated to dryness, and weighed. (ii) Chromatographic Separation of Petroleum Spirit Soluble Fraction. The technique employed in the separation was reported elsewhere (Selucky et al., 1978). The separated saturated hydrocarbon fraction was chromatographed for the individual identification of the heavier components. For the gas-liquid chromatograph analysis of the heavier hydrocarbons, the following conditions were chosen, using 3% SE 30 on 80-100-mesh Chromosorb AW DMGS (9 f t x 1/4 in.) in a glass columr. and flame ionization detector. The column was intially held at 100 "C for 1 min, then temperature programmed to 300 "C at a rate of 8 "C/min, and held at the final temperature until the signal returned to the base line (approximately 25 min). The sample size was 50 pL of sample solution.
Results Results of the Light Gas Evolution. Lignocellulose from elephant grass was pyrolyzed under vacuum conditions, and the light products were analyzed. The major products (in the light gases), identified and analyzed, were CH4, C 8 4 , %Hs, C3H8, c4Hi0, c5Hi0, n-cdfiz, i-C&iz, and n-C6H14. The chromatogram of these compounds is shown in Figure 1. Product yields at different loading conditions for the sample are given in Table I. Product yields at 200, 230, 260, and 300 "C for the samples are given in Table 11, while the apparent first-order rate constants for the product yields at 200,230,260, and 300 "C for the pyrolysis are shown in Table 111. These constants have been estimated based on the equation w = wo(l - e+) where w o is the maximum amount of the product that would be obtained at infinite time, w is the amount of the
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2171 Table 11. Product Yields from the Pyrolysis of Lignocellulose as a Function of Temperature yields," pg
a
t , "C
CH4
CZH4
CZH6
CJ%
C4H10
C5H10
200 230 260 300
0.480 2.94 9.17 74.2
0.121 0.197 0.302 0.502
0.157 0.258 0.402 0.672
0.107 0.175 0.270 0.448
70.4 117.0 172.0 300.0
28.0 45.7 71.0 119.0
i-C5H12
n-C5H1Z
26.2 42.7 66.1 110.0
27.2 46.6 78.6 134.0
n-C6H14
22.7 37.2 57.6 96.4
Weight of sample, 15 mg; pyrolysis time, 2 h.
Table 111. First-Order Rate Constants of CI-CBProducts and Saturated Hydrocarbons from the Pyrolysis of Lignocellulose as a Function of TemDerature
t , "C
CH4
C2H4
CZH6
C3H8
200 230 260 300
0.0557 0.345 1.10 13.3
2.05 3.52 5.84 11.7
1.95 3.36 5.67 11.4
1.87 3.20 5.31 10.4
n-C4H10
1.80 3.12 4.89 10.1
Table IV. Arrhenius Parameters for the Evolution of the Light Gases and the Production of the Saturated Hydrocarbons comud lop A , s-' E., kcal mol-' CH4 6.9 f 0.5 28.7 f 0.3 CZH4
CZH6 C3H8 n-C4H10 C5H10 n-C5H12
i-C5H12 n-C6H14
saturated hydrocarbons
-0.4 -0.3 -0.5 -0.6 -0.5 -0.5 -0.1 -0.4 -1.4
0.1 0.1 0.2 0.3 0.1 f 0.1 f 0.1 f 0.2 f 0.3
f f f f f
9.4 9.5 9.3 9.1 9.3 9.3 10.2 9.4 6.4
0.3 0.2 0.4 0.2 0.3 0.3 0.4 & 0.2 f 0.4
f f f f f f f
C5H10
n-C5Hlz
i-C5H12
n-C6Hlz
saturated hydrocarbons
1.85 3.15 5.27 10.5
1.85 3.16 5.25 10.3
1.54 2.77 5.05 10.2
1.93 3.32 5.55 11.1
3.85 6.02 7.61 13.1
Table V. Effects of Temperature and Sample Loading on Vacuum Pyrolysis of the Lignocellulose temp, "C substrate product" constituents wt, g 200 230 260 300 43 52 124 gaseous products 1.00 35 volatile liquid productsb nonvolatile liquid products (benzene extract) residue
2.00 1.00 2.00 1.00 2.00 1.00 2.00
75 88 200 32
91 93 207 38
110 101 212 48
262 172 356 52
77 82 100 108 845 825 799 652 1648 1620 1578 1274
Weights of products in milligrams. bObtained by difference. C e L
a .7
0 x
m 0
Figure 1. Chromatogram of Cl-cS hydrocarbons from the 200 "C pyrolysis of lignocellulose for 2 h.
product generated at time t , and k is the rate constant. The logarithms of the rate constants of product formation are plotted as a function of 1/T in Figure 2. The resulting preexponential factors and activation energies obtained from the least-mean-squares derivations of the intercepts and slopes are listed in Table IV. The errors quoted are standard deviations; the correlation coefficients averaged 0.99. Results of the Heavier Hydrocarbons and Other Compounds. Lignocellulose was pyrolyzed in an ampule, and the products, identified and measured, were gaseous products, volatile liquid products, and nonvolatile liquid products including tar and undecomposed residue. The results of the product yields a t different loadings of the sample are shown in Table V. Further hydrocarbon-type analysis of the benzene extract gave yields of the fractions of the extract at 200,230, 260, and 300 "C. The results are shown in Table VI. Further analysis of the saturates showed that the dominant
Table VI. Effects of Temperature on the Hydrocarbon-Type Fractions of Petroleum Ether Soluble Fraction from Benzene Extract" temnb "C fraction 200 230 260 300 3.1 4.5 5.4 7.8 saturates monoaromatics 0.0 0.0 0.4 1.0 0.2 0.5 diaromatics 0.0 0.0 9.2 12.4 polyaromatics 7.9 8.7 3.5 3.4 polar fraction 3.6 3.5 3.2 4.5 5.4 7.9 more polar fraction' Weight of lignocellulose = 1 g. grams. Obtained by difference.
Weight of fractions in milli-
portion consists of a homologous series of n-alkanes and minor yield of n-alkenes. The temperature-dependence chromatogram of these components and unresolved peaks of C15-C35hydrocarbons (with CZ5peak as maximum) are shown in Figure 3. The computer GC-mass analyses of these peaks showed that each unresolved peak consists of 80% n-alkane and 20% n-alkenes. Figure 4 shows the distribution pattern of the n-alkenes and n-alkanes, respectively.
Discussion Between 200 and 300 "C, the rate of pyrolysis of lignocellulose is appreciable, and the nature and kinetics of formation of the product gases indicate that the cleavage of unstable side chains and/or functionalities is already taking place. All precaution is taken in solvent extraction steps in order to ensure the removal of the less stable precursors. This treatment also guarantees the removal of all lipids or other higher molecular weight material, which may have become adsorbed in the lignocellulose
2172 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
-4.00 -4.20 1
cn 0
-4.40 200'C
-4.60
'25
-4.80 1.80
1.90 2.00 2.10
Figure 2. Arrhenius plots of the rates of production of the C1-Cs and total saturated hydrocarbons.
network. The excellent reproducibility of the subsequent product yields at 20-300 "C showed that these extractions had effectively stripped the lignocellulose of all the unstable contaminants. Kinetics of the Light Gas Evolution. The product yields of the light gas in Table I seemed to increase fairly uniformly with an increase in the loading of the sample. This result indicated that there was fairly uniform heat distribution within the sample being pyrolyzed. Since the product yields fit the first-order rate equation closely, it is reasonable to assume that the rates of evolution of these products follow first-order kinetics in the temperature range studied. The Arrhenius plots of the evaluated rate constants showed good linear relationships in the temperature range 200-300 "C. The observed first-order kinetics of evolution of C1-C6 hydrocarbons together with the formation of only two alkenes, C2H4and C5HI0,can be rationalized in terms of
Figure 3. Chromatograms of c15-c35 n-alkanes produced at different temperatures (200,230, 260, and 300 "C). First peak from the right is n-Ci5H32, and the last peak is n-C35H72.
the primary cleavage of alkyl side chain, followed by hydrogen abstraction as per the following reaction scheme, lignocellulose R'
-- + + A
+ lignocellulose
R'
RH
radical I radical I1
where R = CH,, CzH5,C3H7, ..., C6H13,while CzH4and C5HI0may be formed from radical rearrangement and decomposition reactions such as the following: RCHzCH2CH2CH2CH2' RCH2CH2CH2' + C2Hd +
RCH2CH2CH2CH2CH2'
-+
R'
+ CH2=CHCH2CHZCH,
The mechanism for the generation of C1-C6hydrocarbons is probably not similar to what is observed in the "high temperature pyrolysis of petroleum asphaltenes" (Ekwenchi et al., 1984). This is because at moderately high temperatures the secondary reactions, namely radical combination-disporportionation reactions, are not important. The rates of product formation for the pyrolysis
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2173 Ion
83.00
amu. f r o m DATA:TPFI.U
5.OE5
5.OE5
4.OE5
4.0E5
3.0E5
3.0E5
I
c u
Ion
0
85.00 amu. f r o m DATA: 1 P F I . D
5 OE5
5 OE5
a
a 4 OE5
4 OE5
3 OE5
3 OE5
2 OE5
2 OE5
1 OE5
1 OE5
0 30
0 40
50
60
Time Imin.)
8000
8000 6000
4000
10000
of the lignocellulose are higher than those of the kerogens (Peters et al., 1981) and asphaltenes (Ekwenchi et al., 1984). The values of the activation energies of these products are substantially lower than those associated with the gas-phase homogeneous decomposition of model organic molecules, and this could be attributed to the occurrence of heterogeneous surface catalyzed reactions of the less stable portions of the lignocellulose. It is observed that these activation energies are smaller than those for asphaltenes decomposition. This suggests that lignocellulose has more less stable portions than the asphaltenes; in other words, the lignocellulose is more sensitive to decomposition in the 200-300 "C temperature range than the asphaltenes. The activation energy for methane generation is 28.7 kcal mol-', which is very high compared to those of the other hydrocarbons, C2-CG. This result seems to suggest that the radical cleavage leading to the methane formation involved much higher activation, which is comparable to that observed in the formation of methane in the thermolysis of Athabasca asphaltenes (Strausz, 1979). It is interesting also to note that butane, which is a major component of the Nigerian natural gas (heating gas), is noticeably high in concentration in the light gases evolved. The result may in the future be examined further to determine if the source of the Nigerian natural gas can be traced to being originated from the diagenesis of the lignocellulosic materials, most probably, from the elephant grass and/or other sources. Kinetics of the Heavier Hydrocarbons and Other Compounds. The result from the gross composition of
the lignocellulose as shown in Table V seemed to indicate that the product yields tend to increase fairly uniformly by doubling the loading of the sample. It also indicated that the rates of formation of these products increased with an increase in temperature from 200 to 300 "C. The C1-C6 hydrocarbon content in the gaseous products was high throughout the temperature range studied (percent by weight of gaseous products: 33 a t 200 "C, 45 a t 230 "C, 58 a t 260 OC and 65 at 300 "C). The rest of the other compounds in the gaseous products are C02, CO, and H20, which were identified but were not measured. There was no detailed identification analysis carried out for the volatile liquid products; however, the determined rates of production of the volatile liquid products were reasonably high (percent by weight of sample: 8.8 at 200 OC, 9.3 at 230 "C, 10.1 a t 260 "C, and 17.2 at 300 "C). From the results in Table VI, the saturated hydrocarbon fraction seemed to increase with an increase in temperature, while the monoaromatic concentrations were low in the temperature range studied. Similar results were obtained in the case of diaromatics. For the polyaromatics, the rates of production were dominantly high, and the trend in the rates' increase was toward the increase in temperature. These results give credence to the speculation that the lignin may have a high portion of polyaromatic moiety linked to the backbone structure of the lignocellulose. The rates of formation of polar compounds remained fairly constant with the temperature increase; however, the rates of formation of more polar compounds which are retained in the column increased with the increase in temperature. The trends in the increase and the amount appeared similar to that of the saturated hydrocarbon production. From this observation, one is tempted to speculate that the decomposition step for these polars might have accompanied the formation of the saturated hydrocarbons. The results from further analysis of the saturated hydrocarbons suggest that the n-alkane-generating portion makes up a substantial part of the lignocellulose structure. Experimental evidence has shown that the oil-generating portions of kerogen and petroleum asphaltenes are probably structurally similar (Bandorski, 1982). The results from the n-alkane-generating portion of the lignocellulose, which is probably similar in structure to kerogen and asphaltenes, go to support the belief that during burial and diagenesis the lignins are converted into kerogen and that during subsequent catagenesis thermal degradation converts a portion of the kerogen into crude oil and hydrocarbon gases. The distribution pattern of these components shown in Figure 3 is noteworthy. An odd-to-even predominance can be observed from both the n-alkanes and n-alkenes, especially in the c1$-c33 range with a maximum a t CZ5. Similar phenomena were observed in thermal alteration studies of lignite and wax esters (Connan, 1974). Esterbound fatty acids yield alkanes having one carbon atom less than the parent fatty acid moiety. The observed odd-to-even carbon number predominance in the n-alkanes may be regarded as a reflection of the even-to-odd predominance of the ester-bound fatty acids in the lignocellulose structure after decarboxylation. The similar distribution pattern for both n-alkanes and n-alkenes suggests that both may have a common precursor. Hence, the observed first-order kinetics of the saturated hydrocarbons together with lower yields of n-alkenes and similar distribution patterns for both n-alkanes and n-alkenes can be rationalized in terms of the primary cleavage of the alkyl side chains of the ester-bound fatty acids linkage in the
2174 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
lignocellulose structure, followed by hydrogen abstraction and decomposition reactions of the alkyl radicals produced: CH3(CH2),*+ COP + radical I11
lignocellulose CH3(CHz),'
+ lignocellulose
CH3(CH2),'
+
-
CH3(CH2),H + radical IV
CH,(CH,),-ZCH=CH2
+H
where n = 14, 15, 16, ..., 134. The activation energy for the generation of these saturated hydrocarbons was close to the energies for CZ-cG hydrocarbons. This further supports the speculation that the primary step in the decomposition must have involved radical cleavage, followed by other radical reactions shown above, leading to the formation of these hydrocarbons. Summary a n d Conclusions 1. Lignocellulose from elephant grass have been pyrolyzed in the temperature region 200-300 "C. 2. The rates of evolution of c1-cG hydrocarbons and that of the formation of C15-C35 hydrocarbons in the saturates fraction follow first-order kinetics, and the Arrhenius plots are linear. 3. The activation energies for product formation, 9.1-10.2 kcal mol-' (c2-c6 hydrocarbons), 28.7 kcal mol-' (CH,), and 6.4 kcal mol-' (saturates fraction), are lower than those associated with the gas-phase homogeneous process, implying that the pyrolytic decomposition of lignocellulose involved heterogeneous surface catalyzed reactions of the lignocellulose. 4. The closeness in the activation energies for the product formation of CZ-cG hydrocarbons and saturated hydrocarbons supports the speculation that the primary step in the decomposition of the lignocellulose must have involved radical cleavage, followed by other radical reactions (abstraction and decomposition). Acknowledgment This work was supported by research Grant UJ/FS/ RG/84-85/013 of the University of Jos, Nigeria. We acknowledge the technical assistance received from both the University of Alberta, Canada, and the British Council, a t the University of Newcastle-upon-Tyne, U.K. Nomenclature A = preexponential factor, s-l
E, = activation energy, kcal mol-' k = reaction rate constant, s-l R = universal gas constant, cal K-' mol-' Registry No. CHI, 74-82-8; C2H4,74-85-1; C2Hs,74-84-0; C,H,, 74-98-6; C4H10, 106-97-8; C5H10, 25377-72-4; CSH12, 109-66-0; iC5H12,78-78-4; C6HI4,110-54-3; lignocellulose, 11132-73-3.
Literature Cited Antal, M. J. Energy from Biomass and Wastes; Klass, D. L., Ed.; Wiley: Chicago, 1978. Appel, H. B. The Production of Oil from Wood Wastes and Fuel from Wastes; Academic: New York, 1977. Appel, H. R.; Fu, Y. C.; Friedman, S.; Yovorsky, P. M.; Nender, L. "Converting Organic Wastes to Oil". US. Bureau of Mines, Report of Investigation 7560, 1971. Bandorski, E. Energy Sources 1982, 6 , 47. Boocock, D.; Mackay. D.; Franco, H.; Lee, F. Can. J . Chem. Eng. 1979, 57, 57. Boocock, D.; Mackay, D.; Franco, H.; Lee, F. Can. J . Chem. Eng. 1980, 58, 466. Connan, J. "Diagenese naturelle et Diagenese artificielle de la materiie organique a elements vegetaux predominants". In Advances in Organic Geochemistry; Tissot, B., Brenner, F., Eds.; Edition Technip: New York, 1974; pp 33-75. Cavelier, J.; Chronet, E. Fuel 1977, 56, 57. Crawford, D. L. Appl. Environ. Microbio. 1978, 35, 1041. Diebold, J. P. "Preliminary Results in the Fast Pyrolysis of Biomass to Lower Olefins". Symposium on Alternate Feedstock for Petrochemicals, ACS Division of Petroleum Chemistry, Las Vegas, 1980. Eager, R. L.; Mathews, J. F.; Pepper, J. M.; Zchdi, H. Can. J . Chem. 1981,58, 2191. Eager, R. L.; Pepper, J. M.; Roy, J. C. Can. J . Chem. 1983,61,2010. Ekwenchi, M. M.; Lown, E. M.; Montgomery, D. S.; Strausz, 0. P. AOSTRA J . Res. 1984, 1, 127. Elliot, D. C. Can. J. Chem. Eng. 1980, 58, 730. Kuestar, I. L. "Liquid Hydrocarbon Fuels from Biomass". ACS Symposium on Biomass as a Non-Fossil Fuel Source, Honolulu, 1979. Molton, P. M.; Miller, H. K.; Russel, J. A.; Donovan, J. M. "Biomass as a Non-Fossil Fuel Source". ACS Symp. Ser. 1981, 144, 137. Peters, K. E.; Rohrback, B. G.; Kaplan, I. R. AAPG Bull. 1981,65, 688. Roy, C.; Chronet, E. Proc. World Congress Chem. Eng., 2nd 1981, 2, 315. Selucky, M. L.; Chu, V.; Ruo, T. C.; Strausz, 0. P. Fuel 1978,57, 9. Strausz, P. 0. Final Report AOSTRA/University Agreement 29, 1979. Waterman, W. W. Symposium on Clean Fuels from Biomass, Sewage, Urban Refuse and Agricultural Wastes, Orlando FL, 1976. Received for review November 19, 1987 Revised manuscript received June 23, 1988 Accepted July 12, 1988
