AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 1, NUMBER 2
MARCH/APRIL 1987
@Copyright 1987 by the American Chemical Society
Art i c 1es Molecular Characterization of the Pyrolysis of Biomass. 1. Fundamentals Robert J. Evans and Thomas A. Milne* Solar Energy Research Institute, Golden, Colorado 80401 Received-September 9, 1986. Revised Manuscript Received November 3,1986 The technique of molecular-beam, mass spectrometric (MBMS) sampling is applied to the elucidation of the molecular pathways in the fast pyrolysis of wood and its principal isolated constituents. The goal is the optimization of high-value fuel products by thermal and catalytic means. The positive-ion mass spectra shown are obtained from real-time, direct sampling of light gases, reactive intermediates, and condensible vapors simultaneously. The cellulose, lignin, and hemicellulose (e.g., xylan) components of wood pyrolyze largely to monomer and monomer-related fragments and give characteristic mass spectral signatures. Whole wood appears to behave as the s u m of its constituents, with few if any vapor species derived from interaction of the main polymer constituents. An important interaction, however, is the influence of mineral matter in the wood on the carbohydrate pyrolysis pathways. Vapor phase cracking of the primary products proceeds through a stage of light hydrocarbons and oxygenates to the ultimate formation of aromatic tars and H2, CO, COz,and H20. These steps are illustrated and discussed. Consistent with these observations, a relatively simple pyrolysis reaction scheme is proposed.
Introduction The pyrolysis of lignocellulosics has been studied for decades, much of the earlier work being motivated by concern for fire suppression and interest in chemicals from ~ 0 o d . lMore ~ recently, the desire to optimize processes for conversion of biomass and wastes to heat and to gaseous and liquid fuels has led to many more studies of pyrolysis, often using new techniques and instrumentation. Interest continues in fire research and most recently, in "nuclear winter" related smoke generation. Several current comprehensive reviews of the pyrolysis of wood and its major constituents have been published,&" and two spe(1) Roberta, A. F. Combust. Flame 1970, 14, 261. (2) Beall, F. C.; Eickner, H.W. Res. Pap. FPL-For. Rod. Lab. (U.S.) 1970, FPL 130. (3) Shafiideh, F. Adu. Carbohydr. Chem. 1968,23,419-474. (4) Gooe,A. W. In Wood Chemistry, 2nd ed.;Wise, L. E., Jahn, E. C., Ede.;ACS Monograph 97; American Chemical Society Washington, DC, 1952; Vol. 2, p 826. (5) Broido, A.; Kilzer, F. J. Fire Res. Abstr. Rev. 1963, 5, 157-161. (6) W e , T. A. In A Survey of Biomass Gmification;Reed, T. B.; Ed.; Noyea Data Corp.: Park Ridge, NJ, 1981; Vol. 2, Chapter 5. (7) Soltee, E. J.; Elder, T. J. In Organic Chemicals from Eiomuss; Goldatein, I. S., Ed.; CRC Boca Raton, FL; 1981 pp 64-99.
cialists meetings covered major aspects of pyrolysis12J3so no attempt will be made in this paper to summarize the whole field. (Excluded from this discussion are the complex, condensednphase, direct liquefaction processes that combine pyrolysis with catalysts, high pressure, reactive environments, and long residence times; see ref 14-17 for reviews of this chemistry.) (8) Shafizadeh, F. J. Aml. Appl. Pyrolysis 1982, 3, 283-305. (9) Antal, M. J. Jr. Adu. Solar Energy 1982,1, 61-112. (10) Antal, M. J. Jr. Adu. Solar Energy 1986,2, 175-255. (11) Graham, R. G.; Bergougnou, M. A.; Overend, R. J. Anal. Appl Pyrolysis, 1984, 6, 95-135. (12) Diebold, J. P. Proceedings of Specialists Workshop on Fast Pyrolysis, SERI/CP-622-1096; Solar Energy Research Institute: Golden, c o , 1980. (13) Overend, R. P., Milne, T. A., Mudge, L. K., Eds. Fundamentals of Thermochemical Biomass Conuersion; Elsevier Applied Science: New York, 1985. (14) Molton, P. M.; Demmitt, T. F. Eattelle Pac. Northwest Lab., [Rep.] BNWL 1977, BNWL-2297. (15) Chomet, E.; Overend, R. P. Bioenergy 84 1985, 3, 276296. (16) Chomet, E.; Overend, R. P. In Fundamentals of Thermochemical Biomass Conuersion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: New York, 1985; pp 967-1002. (17) Moffatt, J. M.; Overend, R. P. Biomass 1986, 7, 99-123.
0887-0624/87/2501-0123$01.50/00 1987 American Chemical Society
Evans and Milne
124 Energy & Fuels, Vol. 1, No. 2, 1987
Many techniques have been used to study biomass pyrolysis under conditions chosen to yield both practical and scientific insight into kinetics and product yields. A large number of past studies have measured weight loss by TGA and related techniques1*or have collected final gaseous, liquid and solid products for analysis and to close mass balances.lg Conditions have ranged from close simulation of the particle size and environments of practical devices, to the study of milligram, or even microgram, samples in vacuum (as in analytical pyrolysis mass spectrometry20). The behavior and properties of the solid residue have been probed in a variety of ways.21 In only a few studies have the intermediate steps in gas phase pyrolysis been o b ~ e r v e d . ~ *These ~ ~ - ~studies ~ concentrated mainly on the light-gas end products of primary vapor cracking. The focus of the work described here is on the complex, organic, vapor product slate in both primary and vapor phase pyrolysis of lignocellulosics, as observed by a unique, direct-sampling, mass spectrometric technique. The goal is to extract, in real time, samples of gases and vapors directly from pyrolyzing systems at ambient pressure, rapidly quench these samples and, introduce them into the ion source of a mass spectrometer (as a “universal detector”), without allowing wall collisons or condensation of vapors. What is sought are the molecular pathways the biomass vapors follow, as a function of temperature, residence time, environment,and other variables of practical interest. Since the emphasis of our work has been on the optimum conditions for the production of high yields of condensed oxygenates or hydrocarbon gases, the conditions chosen have been those of moderate-to-fast heating (degrees per second rather than degrees per minute) where char, water, and C02 yields are minimized. This paper attempts to summarize the insight we have gained in several years of study of the pyrolysis of wood and its main components under “fast pyrolysis” conditions, with references given to more detailed discussions in other publications.
Experimental Section The experimental approach to achieving the sampling goal stated above is to couple extractive sampling (as a free-jet through a small orifice) with rapid transition to molecular flow, collimation
of a molecular beam, and line-of-sight introduction of the molecular beam into the ion source of a mass spectrometer. The apparatus to accomplish this is shown in Figure 1. Vapor from a pyrolysis reactor flows over and into a sampling orifice. The initial expansion is nearly adiabatic and is isentropic, with the consequence that extreme collisional and internal energy state cooling can occur.25 With a low enough pressure in the first vacuum stage, and proper placement of the second slit (skimmer), the supersonic flow enters the skimmer without shock formation. Rapid quenching and formation of an intense molecular beam (18) Antal, M. J., Jr.; Friedman, H. C.; Rogers, F. E. Combust. Sci. Technol. 1980,21,141-52. (19) Nunn, T. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Ind. Eng. Chem. Process Des. Dev. 1985,24, 836-844. (20) Meuzelaar, H. L. C.; Haverkamp, J.; Hileman, F. D. Pyrolysis Mass Spectrometry of Recent and Fossil BiomateriaIs; Compendium and Atlas; Elsevier: Amsterdam; 1982. (21) Sekiguchi, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1984, 29,
1267-1286. (22) Ekstrom, C.;Rensfelt, E. In Proceedings of Specialists Workshop on Fast Pyrolysis, SERI/CP-622-1096; Solar Energy Research Institute: Golden, CO, 1980; p 303. (23) Diebold, J. P. M.S. Thesis T-3007, Colorado School of Mines, Golden, CO. (24) Steinberg, M.; Fallon, P. T.; Sundaram, M. S. Proceedings of the 1985 Biomass Thermochemical Conversion Contractors’ Meeting, PNL-
SA-13571, CONF-8510167; NTIS: Springfield, VA, 1986; p 15. (25) Milne, T. A.; Green, F. T. J. Chem. Phys. 1972,56, 3007.
Quadrupole MS Electron beam
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Heater
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Figure 1. Schematic of pyrolysis vapor generator coupled to a molecular-beam mass spectrometer sampling system. are achieved. The molecular beam intercepts a low-energy electron beam (15-22.5 eV) of a quadrupole mass spectrometer, yielding a positive ion mass spectrum. The details of such sampling have been discussed sin several papers and review^.^^^' The main features for our sampling applications are as follows: collisions are minimized as the gas cools so that even highly reactive and condensible species can be preserved; rotational and vibrational cooling of molecules occurs, leading to less ionization fragmentation in the mass spectra; light gases and heavy vapors are sampled simultaneously and in real time; the mass spectrum provides a “fmgerprint”that gives a rapid, semiquantitative picture of the complex vapor; sampling can occur from ambient atmosphere pressures and temperatures that closely simulate the environment of pyrolyzers, gasifiers, or combustors. The limitations of this technique will be discussed as results are presented. A number of reactor systems for pyrolyzing small samples of biomass have been interfaced with the free-jet, molecular-beam mass spectrometer (MBMS) sampling system. These are placed in the desired proximity of the sampling orifice, as illustrated in Figure 1for a two-section furnace source. In early work an open, premixed, H2-02-He or -Ar flat-flame burner was used to create a laminar flow of hot steam-He or -Ar into which the sample to be pyrolyzed was placed. By varying the placement of the burner and sample relative to the sampling orifice, one could quickly survey both the primary species and the secondary reactions of primary vapor species in the gas phase. Results from this system have been p ~ b l i s h e d . ~ ~ * ~ ~ - ~ ~ (26) Milne, T. A.; Soltys, M . N. J. Anal. Appl. Pyrolysis 1983, 5, 93-110,111-131 (93-131). (27) Milne, T. A.; Beachey, J. E. Combust. Sci. Technol. 1977, 16, 123-38,139-52. ‘(28)Knuth, E. L. Appl. Mech. Rev. 1964,17, 751. (29) Anderson, J. B.; Andres, R. P.; Fenn, J. B. In Molecular Beams; Ross, J., Ed.; Wiley: NY 1966;pp 275-317. (30) Ashkenas, H.; Sherman, E. S. Rarefied Gas Dyn. 1966,2,84-105. (31) Steams, C. A.; Kohl,F. J.; Fryburg, C. G.; Miller, R. A. NBS Spec. Publ. (US.)1978,No. 561, 303. (32) Milne, T. A.; Soltys, M. N. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: New York, 1985; pp 361-383. (33) Evans, R. J.; Milne, T. A.; Soltys, M. N.; Schulten, H. R. J. Anal. Appl. Pyrolysis 1984,6, 273-283.
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987 125 olution from total ion current curves such as that shown in Figure
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Figure 2. Example of data from a typical batch pyrolysis of a small sample of pine wood (30 mg): (a) total ion count to time (8); (b) selected ion profiles from extractives (m/z 226), lignin (m/z 180 and 138), hemicellulose ( m / z 114 and part of m / z 43), and cellulose ( m / z 43); (c) average spectrum with background correction over the whole pyrolysis interval. In order to have a better defined temperature history for the secondary cracking and to operate at pyrolysis temperatures lower than about 800 "C, the burner system has been replaced by single or two-section resistance-heated flow reactors as shown in Figure 1. This simple system allows preheating of gas to the desired temperature in the lower section, insertion of a sample quickly into the convective and radiant heating environment in the middle of the reactor, and control of the vapor temperature and residence time in the upper section where secondary gas phase reactions occur. Antal was perhaps the first to use separated zones to isolate primary from secondary reactions? In work to date, we have only used captive pieces (or samples in boats) as the source of pyrolysis vapors, in order to clearly separate the primary pyrolysis region from the secondary region by preventing the solid from entering the secondary region as in entrained flow pyrolysis. To simulate entrained flow pyrolyzers and to achieve the extraordinary heating rates experienced by micrometer-sized powders injected into hot gases (>lo3 "C/s), powder-laden gas flows could be sampled as a function of distance from injection, as was done in pulverized-coal flames in past work.n For primary pyrolysis work, the upper section of the reactor is removed so that the orifice can be placed quite close to the pyrolyzing surface, where temperatures are low due to the endothermic nature of fast pyrolysis and secondary reactions are minimized. No direct measurement of sample heating rate or sample temperature was made. Only the temperature of the flowing gas into which the sample was placed was measured. The effect of surrounding gas temperatures, in the range 400-500 "C used t o observe primary spectra, was negligible. Where heating rates are quoted they are estimated from the rise-time to peak vapor ev(34) Evans, R.J.; Milne, T. A.; Soltys, M. N. J.Anal. Appl Pyrolysis 1986,9, 207-236.
Temperature profiles in the upper, vapor phase cracking region were measured by using doubly shielded thermocouples to establish rough constancy to f10 OC. This was felt to be more than adequate for the qualitative deductions reported here. Residence times were simply calculated from bulk flow a t the temperatures used and the tube cross section. Cooling effects at the upper end of the reactor were minimized by inserting a small-diameter extractor tube about 5 cm into the reactor to minimize residence time in that region. For the secondary cracking experiments, both the lower and upper section temperatures were set at the same value to increase the isothermality of the upper section. Past work with open-flame heat sources has shown that the product slate emerging from samples placed in hot gases is little changed over the surrounding gas temperature range of 400-900 "C. This is attributed to the endothermic nature of primary pyrolysis. In addition to the pyrolysis of small inserted samples, small conversion devices (gasifiers, combustors) can be directly interfaced with the sampling orifice. Larger devices, such as wood stoves, would require the use of heated probes connecting the sampling orifice to the region to be sampled, due to the geometric constraints of the rather open sampling stages needed for undisturbed, free-jet expansion to molecular flow. Data acquisition is continuous, through digitization of electron-multiplier signals from the arrival of positive ions and programmed storage in an IBM PC computer. The data system used was made by Teknivent= primarily for GC-MS applications, but it is well suited for our purposes. We repetitively scan (typically one 300 amu scan/s) during the evolution of a pyrolysis wave from samples of 10-1000 mg inserted into the hot environment. The stored spectra can be manipulated to give average spectra, subtracted spectra, or time evolution of different masses. In addition, the individual ion data for groups of samples can be processed by using available data analysis programs that allow flexible graphical display as well as multivariate analysis of the data. Examples of the kinds of data that emerge in a batxh pyrolysis experiment are given in Figure 2. The upper curve shows the total mass spectral ion current, which profiles the duration of the pyrolysis wave during a typical fast pyrolysis (>30 "C/s) experiment. The spectrum is the average of 50 consecutive 1-s scans during the fast pyrolysis of the sample, with the background signal due to electronic noise and ion source gases subtracted. The middle curve shows single-ion monitoring of six selected masses during pyrolysis. A word of caution is necessary a t this point as the reader interprets the data to be shown below. A number of variables can be manipulated to alter the appearance of the positive ion mass spectra. Foremost are the electron energy, which affects fragmentation and relative parent-ion sensitivities, and the tuning of the q ~ a d r u p o l e where , ~ ~ tradeoffs between mass resolution, intensity, and high and low mass emphasis are routinely made. In addition, the free-jet sampling process causes a "massseparation" effect, which favors heavier molecules by approximately the first power of the molecular eight.^' Also the low electron energy, used in most of our work to minimize parent-ion fragmentation, leads to overemphasis of molecules with low ionization energies (e.g., aromatics and olefins) vis-a-vis permanent gases and saturated hydrocarbons (e.g., CHI, HsO). Without tedious calibrations using standard additions of known compounds under actual pyrolysis-free-jet conditions, the results of most significance are the kinds of species observed and their relative change with pyrolysis conditions, rather than their absolute values. Thus the approach taken here is complementary to other more easily quantifiable techniques such as GC, GC-MS, FTIR, condensation of products followed by chemical analysis, etc. Its outstanding advantages are the rapidity of data acqui(35) Teknivent (1984) Model 1050 mass spectrometer data system, Teknivent Corp., St. Louis, MO 63146. (36) Extranuclear (1979) quadrupole mass filter Model 4-162-8, Extranuclear Laboratories, Inc., Pittsburgh, PA 15238. Brewer, J.; Milne, T. A. J. Chem. Phys. 1964, 40, (37) Greene, F.T.; 1488-1495.
126 Energy & Fuels, Vol. 1, No. 2, 1987
Evans and Milne
sition, the fact that all volatile species can be directly sampled pyrolysis takes place and the varieties of biomass. Thus from their reactive environment,and the nearly universal detection various mixtures of primary and secondary organics are capability of mass spectrometry. Because no prior separation is obtained, depending on the thermal history and environmade, as in GC-MS,isomers can seldom be distinguished and ment of the pyrolysis. severe fragmentation can obscure parent molecules. FragmenMass spectra of “primary” pyrolysis products from a tation can be used to distinguish compounds with the same number of samples of whole biomass and its major comnominal molecular ion. Ethylene, for example, has a fragment ponents are shown in Figure 3 and a listing of inferred ion at mf z 27, which can be used to determine the relative conchemical species is given in Table I. By “primary spectra” tribution to mlz 28 of ethylene and CO. These disadvantages we mean observed spectra that are substantially free of are in part offset by the ability to observe 100% of pyrolysis vapors that by most techniques are often only 30-70% chromatosecondary gas phase cracking products as inferred from graphable when the condensed vapors are r e v a p o r i ~ e d . ~ ~ ~ ~ the secondary cracking studies discussed below. Cracking Finally, mass spectral “fingerprints,”particularly if fragmenof primary products within the pyrolyzing particle can also tation is minimized, are more intuitively interpretable than gas occur during their escape to the surface. Over the dichromatograms, since the displayed axis is molecular weight and mensions of particles typically pyrolyzed (1-6 mm narresolution is quantized and roughly constant. In general the m rowest dimension) only small changes in relative mass spectra, vis-a-vis gas chromatograms, are simplified by the spectral peak heights occur (less than a factor of 1.5 for overlapping of parent ions of isomers, but are complicated by almost all peak ratios). Such changes have no effect on virtue of fragment ions. Ultimately it is probably not affordable, the qualitative discussion to follow. For large particles, or desirable, to attempt to routinely quantify the hundreds of of industrial interest, internal cracking would become imminor species known to be involved in pyrolysis. What will be more important is the behavior of major products or functional portant and should be quantified. classes of compounds, which affect product properties and define In our tabulation in Table I, we have restricted the list mechanistic pathways. The rapidly emerging techniques of to what we believe are primary pyrolysis products from multivariate statistical analysis, such as factor analysis,41combined wood and its separated components, as revealed by parent, with the ready acquisition of pyrolysis “fingerprints”may be the or prominent fragment ions from fast-pyrolysis mass preferred strategy for exploratory research. spectra such as shown in Figure 3, and from recent analyses of primary products from collected pyrolysis oi1s.38939v40*45 Results and Discussion Many confirming reports of these species exist in the literature7J4s46,47No species are observed from wood that A number of reviewers of biomass pyrolysis have postulated more or less complex reaction schemes to account appear to derive from a chemical interaction of the organic for slow and fast pyrolysis and primary and secondary gas components, though proportions may change due to matrix phase pathways. These range from simple s c h e m e ~ ~ * ~ J ~ ~ -effects and inorganic interactions. This is in agreement to complex, multipath s c h e m e ~ . ~ * ’ ~ J ~ with Goos’ observation^.^ The references in Table I cite It has been a major goal of our work, and of this paper, independent, chemical confirmation of the compounds to add molecular detail to the substances variously labeled listed. as “volatiles”, “monomers”, “tar”, “anhydrosugars”, Almost all of the even-mass ions observed in our mass “primary tars”, “low molecular weight liquids”, spectra can be related to observed chemical species, though ”oxygenates”, “transient oxygenated fragments”, ambiguities are inherent in our mass spectra due to frag“secondary tar”, “vapor-phase-derived tar”, “refractory mentation, as the case of levoglucosantypifies.% A number tars”, ”refractory condensibles”, “secondary gases”, and of odd-mass ions (due to fragmentation since nitrogen is “pitch” and the pathways for their formation. One can virtually absent in wood) are prominent and are useful for appreciate the need for a standardizations of nomenclature pattern recognition correlations. Of special note are m/z for pyrolysis products. The simple use of “oil” and “tar” 43’ (CH30+),a common fragment from aldehydes, ketones, is ambiguous for biomass-derived oxygenates, however, due and other carbonyl compounds, and m/z 57 and 73, which to the traditional use of those terms in the fossil literature. are common fragment ions of carbohydrates like levoNature of Primary Products from Biomass Pyroglucosan. lysis. The number of chemical species that have been Through spectra in this paper are shown only to 300 reported from wood pyrolysis and wood distillation is m u , we have carried out mass spectral scans to 1400 m u . enormous. As an example Goos4lists 231 compounds that With the quadrupole adjusted to greatly over represent have been found in the liquid products from the dehigh masses (as judged by the standard fragmentation structive distillation of wood. Soltes and Elder’ in a recent pattern of perfluorotributylamine) levoglucosan oligomers review, tabulate about 230 compounds identified in up to the octamer and lignin polymers beyond the trimer “pyroligneousacid” and “tar” products from softwood and were seen from cellulose and lignin, respectively. On the hardwood pyrolysis. Part of the reason for these large basis of calibrations at lower mass and comparisons of our numbers is the variety of conditions under which wood spectra for levoglucosan and lignin with field ionization r e ~ u l t s ,the ~ ~ higher , ~ ~ molecular weight species are estimated to constitute less than a few percent of the vapor. (38) Elliott, D. C. Final Report on IEA Co-Operative Project D1, The main processes leading to the primary pyrolysis Biomass Liquefaction Test Facility Project; Pacific Northwest Laboraspectra for carbohydrates and lignin have been extensively toires: Richland, WA, 1983; Vol. 4. (39) Beaumont, 0. Wood Fiber Sci. 1985,17, 228-239. discussed in two specialized report^^^.^ as has the special (40) Menard, H.; Belanger, D.; Chauvette, G.; Gaboury, A,; Khorami, A synopsis of these mechanisms problem of levogluco~an.~~ J.; Grise, N.; Martel, A.; Potvin, E.; Roy, C.; Langlois, R. Fifth Canadian follows. Bioenergy R&D Seminar; Hasnain, S., Ed.; Elsevier Applied Science: New York, 1985; pp 418-439. Primary Pyrolysis Pathways. There have been two (41) Wjndig, W.; Meuzelaar, H. L. C. Anal. Chem. 1984,56,2297-2303. main theories of cellulose pyrolysis: free radical and (42) Kilzer, F. J.; Broido, A. Pyrodynamics 1966, 2, 151-163. heterolytic depolymerization. Both of these theories have (43) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979,23, 3271-3280. (44) Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. J. Ind. Eng. Chem. Process Des. Deu. 1982,21, 457-465. (45) Piskorz, J.; Radlein, D.; Scott; D. S. J.Anal. Appl. Pyrolysis 1986, 9, 121-137. (46) Fullerton, T. J., Franich, R. A. Holtforschung 1983,37,267-269.
(47) Obst, J. R. J. Wood Chem. Technol. 1983, 3, 377. (48) Evans, R. J. Direct Mass Spectrometric Studies of the Primary Pyrolysis of Carbohydrates, SERI/TR-234-3061; Solar Energy Research Institute: Golden, CO, 1986.
Pyrolysis of Biomass 100
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Energy & Fuels, Vol. 1, No. 2, 1987 127
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m/z m/z Figure 3. Characteristic mass spectral patterns of primary pyrolysis products for several whole biomass samples and for separated constituents of biomass. Approximately 30 mg of powdered sample was suspended in a steel boat (0.05 mm thickness) in flowing 550 O C helium, giving an estimated heating rate of 30-50 OC/s. The particle size ranged from 50 to 250 pm. The gas phase residence time before free-jet sampling was approximately 75 ms. The actual solid temperature is unknown and less than that of the surrounding 550 "C helium due to the endothermic nature of the pyrolysis. Product evolution occurred over 15-25 s depending on the sample.
some experimental support, but the evidence is inconclusive. Discussions of this debate can be found in recent articles by AntallO and Evans48and are only briefly summarized here. A free-radical mechanism has been proposed by Kislitzyn et al.49on the basis of experiments with dinaphthylphenylenediamine as a free-radical scavenger. When cellulose was pyrolyzed with 5 mol % of this material, the levoglucosan yield was reduced from 30 to 5 wt %. Shdizadeha has proposed a heterolytic mechanism on the basis of the work with phenylglucosides where the nature of the substituents on the phenyl ring dramatically affected transglycosylation. The influence of electron withdrawal or donation led to the hypothesis that the reaction is not a free-radical process but rather involves heterolytic cleavage of existing glycosidic acid bonds via a transglycosylation process involving nucleophilic displacement of the glycosidic groups by one of the free hy(49) Kislitayn, A. N.; Rodionova, Z. M.; Savinylch, V. I.; Guseva, A. V. Zh. Prikl. Khim. (Leningrad) 1971,44, 2518-2524.
droxyl groups. The results of the molecular-beam mass spectrometry studies have been reported in depth48and favor a concerted displacement reaction that is similar to Shdizadeh's proposed mechanism; however, unambiguous proof of intermediates is still lacking. The primary pyrolysis spectrum of pure cellulose is shown in Figure 3. The major peaks a t m / z 162 and 144 (largely an ionization fragment ion) are due to levoglucosan (XXIX; Chart I), which can be obtained from pure cellulose in yields of up to 60%.s Work with chemical ionizat i ~ has n ~shown ~ that a major part of mlz 57,60,73, and 98 ions are also E1 fragment ions from levoglucosan. The dominance of this one product is due to the structural homogeneity of cellulose and the possibility of its undergoing intramolecular condensation in the sequential depolymerization of the glycosidic units (Scheme I). This mechanism allows complete devolatilization, with char yields of less than 1% observed in fast pyrolysis in pure cellulose. The importance of cellulose structural features such as crystallinity and degree of polymerization (DP) has
Evans and Milne
128 Energy & Fuels, Vol. 1, No. 2, 1987
Table I. Major Ions in Mass Spectra of Wood and Compounds Reported in Primary Oils and Gases
ion, m/z 16
18 28
32 44 46 58 60
68 72 74
76 82
84 86 88
94 96 98 100
102 108 110 112
114 116 120 122
124 126 132 138 146 150 152 154 162 164 166 168 178 180
formula
structure
likely product
methane water carbon monoxide co ethene CH4H2 methanol MeOH carbon dioxide OCO acetaldehyde OCHMe formic acid HCOOH 2-propanone (acetone) MeCOMe acetic acid H02CMe hydroxyacetaldehyde (glycoaldehyde) HOCHzCHO methyl formate MeOCHO furan I 2-butanone EtCOMe acrylic acid CH2-CHCOOH 1-hydroxy-2-propanone (acetol) HOCH2COMe propanoic acid HO2CEt hydroxypropanal HOCHzCHzCHO methyl acetate CH3COOCH3 glycolic acid HOCH2COOH 2-methylfuran I1 cyclopentenone I11 cyclopentanone IV 2(5H)-furanone V 2,3-butanedione MeCOCOMe crotonic acid MeCH=CHCOOH butyrolactone VI butanoic acid H02CEtMe 1-hydroxy-2-butanone MeCh2COCH20H phenol PhOH dimethylcyclopentene VI1 furfural VI11 2-methyl-2-cyclopentene-1-one IX furfuryl alcohol X 5-methyl-2(3H)-furanone (a-angelicalactone) XI 3-methyl-2(3H)-furanone XI1 valerolactone XI11 2,3-pentanedione MeCOCOEt pentanoic acid (valeric/isovaleric) CH,(CH*)&OOH o-cresol CH~C~HIOH m/p-cresol 5-methylfurfural XIV dihydroxybenzenes (catechol, hydroquinone, resorcinol) 2-hydroxy-3-methyl-2-cyclopentene-l-one dimethylcyclopentanone 3-hydroxy-2-penteno-1,5-lactone hexanoic acid 1-acetyloxy-2-propanone vinylphenol dimethylphenols ethylphenol benzoic acid 2-methoxyphenol (guaiacol) trimethylcyclopentenone 5-(hydroxymethyl)-2-furfural 2-methyl-3-hydroxy-4-pyrone 1-hydroxy-2-propanone acetate 4-methylguaiacol XXII MeCOO(CH2)2(CO)CHzOH 1-hydroxy-2-butanone acetate p-vinylguaiacol XXIV coumaryl alcohol xxv 4-ethylguaiacol XXVI vanillin XXVII 2,6-dimethoxyphenol (syringol) XXVIII levoglucosan XXIX isoeugenenol xxx eugenol 1-(4-hydroxy-3-methoxyphenyl)ethanone XXXI 4-methy1-2,~i-dimethoxyphenol XXXII coniferyl aldehyde XXXIII coniferyl alcohol XXXIV vinylsyringol xxxv XLV a-D-glucose
MeH HOH
ref to chem anal. of oils
major ionh
in product at >0.1%'
X
X
f
X X
X X X
f f
X X X X
X X X X X X
X
X
a, b, d , e
X X X X X X X X X X X X X X X
f
a, b, e
e e c-e c, e e
X
a
X X X X
d, e e c-e a, d
X
a c , d, f c, d
X X
a c, e, f d, e d, e a, e
X
b, c, e
X
a
g
X
c, d a
g X
c, d
X
d
X X X
a
X X X X
X
a-e a
g d e a, b, e
e i?
X
g X
X
a
g X X
a, b, d , e
c , d, f g
X X X
X X X
d
a-d
X
X X
e
X X X X X X
X
X
c-e a, b
a, b
X
X
e a-f
e b
X
X X
a-f a-f a g a, c-e
X X X X X X
X
b, e b-e e
X
X
X X
a-f
d a a, e a
g X
g d
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987 129 Table I (Continued)
ion, mlz
182 194
formula C9H1004
cl&140s C11H1403 C10H1004
196 208
210 272
c181204 C11H1204 C11H1404
C16H16O4
structure XXXVI XXXVII XKXVIII XXXIX XL xL1 XLII XLIII
major ion"
likely product syringaldehyde 4-ethylsyringol 4-propenylsyringol ferulic acid (4-hydroxy-3,5-dimethoxyphenyl)ethanone sinapyl aldehyde sinapyl alcohol
ref to chem anal. of oils
in product at >0.1%'
a-d a a
X
X X
X
g
X
a
X X
g
X
k-
X
'Reference 38. GC/MS and GC of a vacuum pyrolysis oil from the University of Sherbrooke (poplar). *Reference 38. A flash pyrolysis oil from the University of Waterloo fluidized bed (poplar). 'Reference 40. A variety of analytical techniques applied to a University of Sherbrooke vacuum pyrolysis oil (poplar). dReference 40. A flash pyrolysis oil from the University of Waterloo (poplar). eReference 39. GC/MS of oils from a 1-atm pyrolysis of powdered beechwood. /Reference 45. GC analyses of hybrid poplar, pyrolyzed at 504 O C in a fluidized bed. #Identified in other studies. "Under arbitrary tuning of mass spectrometer, as used in Figure 2. 'As percent of water-free, organic oil for ref a-d and percent of dry wood for ref e and f.
Scheme I
Scheme I1
R-
I
R-
R = CELLULOSE CHAIN HOC H,
RR-
k:
OH
0 11
I
HOCH&H
c=c
,CH -CH 11
R-0'
%H
\
"9 R-
*
@OH
0
I
/ been discussed in past work,1° but in general these effects are not large32in fast pyrolysis compared to the effect of inorganic impurities, as discussed below. Several workers have shown, by kinetic analysis of thermal gravimetry and by differential scanning calorimetry experiments, the existence of a low DP stage42or "active stage, which corresponds to a decrease in DP from 100&2000 to 200 at temperatures between 220 and 250 "C. Thermogravimetric studies (TGA) have shown that cellulose undergoes a rapid weight loss beginning around 280 "C with an inflection point around 350 0C.8 At this temperature cellulose undergoes transglycosylation to form levoglucosan in high yield~.~7& There are several possible transformations that could contribute to the rate-limiting step: the decrease in DP, the breaking of inter- and intramolecular hydrogen bonding to form a less rigid structure (i.e., a glass transition zone), the formation of a reactive intermediate by the cleavage of the glycosidic linkage (i.e., a free radical or ionic species), or the interconversion from the chair to the twist conformation so that the carbon in the 6-position will be in the axial position as it is in levoglucosan.50 It is possible that (50) Cemy, M.;Stanek, J., Jr. A d a Carbohydr. Chem. Biochem. 1977, 34, 23-177.
CROSS-LINKING
CARBONYL COMPOUNDS
BETWEEN CHAINS
more than one of these transitions are important in the rate limiting step and that they are all enhanced at higher temperatures. In the pyrolysis of most biomass materials, however, the levoglucosan yield is low despite cellulose concentrations of approximately 50% .a The treatment of cellulose with small amounts of alkali material can drastically change the product slate as shown in Figure 3. The addition of 0.1 w t ?& potassium or sodium as the carbonate or hydroxide inhibits the formation of levoglucosan and leads to a different product state composed of furfural derivatives (mlz 126, 110,96), low molecular weight carbonyl compounds represented by mlz 43, a common fragment ion of carbonyl compounds such as acetaldehyde and acetol, high molecular weight condensed species (in relatively low abundance), and an increase in char yield from 1to 10%. Our work over the past several years has led to the speculative mechanism shown in Scheme 11for the alkali-metal-catalyzed pathway. With the disruption of intramolecular condensation (transglycosylation), glycosidic rupture becomes more important with dehydration to form carbonyl groups, double bonds, and substituted furans. Piskorz et
Evans and Milne
130 Energy & Fuels, Vol. 1, No. 2, 1987 Chart I
b II
I
IV
111
VI1
V
0 & C H .
pn
\ I Vlll
n3cfY
X
IX
n
q
n
8
n
,
XIV
XI1
XI
d
q qn2 0 hMe
@a;
QOMe
xv
XVll
XVI
XVlll
cn=cn,
"OH"QH
XIX
cn=cH-cn,on
XXI
HrCH1
Me@Me
@Me no
Me
no
0
XXll
&
QOMe
me
xxx
XXlX
xxv
XXlV
XXlll cn=cn-cn,
XXXl
XXVlll
XXVl 0 II
cn=cn-cH
J+ Me
Me
@Me no
Me
XXXll
xxxv
XXXIV
XXXlll
Me
no
HO
0
0 II
Me0
0
cn=cn-cH, Me
M e 0QMe
no
XXXVI
XXXVlll
II
cn=cn-coon
00. no
XXXIX
cn=w-cn
M
Me
XL
M e 0@Me no
XLI
CH=CH-CHPH
no
XLll
en &
noc n
w&n,
n
ns
XLlll
-%
XLlV
XLV
XLVl
al.45proposed that the monomer unit of cellulose decomposes to two-carbon and four-carbon fragments with the two-carbon fragment rearranging to give high yields of hydroxyacetaldehyde (up to 17% yield). Hydroxyacetaldehyde may be the major product of the glycosidic rupture pathway accounting for m/z 60 and fragment ions a t m / z 31 and 32 in the spectra. Piskorz et al. did not address the effect of alkali-metal catalysis in their work. The biomass samples shown in Figure 3 show the distribution of the two major carbohydrate mechanisms. The distribution of the carbohydrate peaks in these whole biomass samples shows that the levoglucosan yield is low, as judged by the relative amounts of m/z 43 and 126 from the glycosidic rupture mechanism and m / z 60 and 144 from the transglycosylation mechanism. Only the douglas fir has a major peak a t m / z 144, showing the relatively minor importance of levoglucosan in the other biomass samples relative to ash-free cellulose. However, even with yields of levoglucosan less than 5%, it is still one of the major single pyrolysis products.
These two major mechanisms for cellulose are also of importance for the other carbohydrates in the hemicellulose fraction of biomass. The pyrolysis of larch wood xylan is shown in Figure 3. Xylan is a major hemicellulose constituent in hardwoods that is a copolymer with glucuronic acid and is commonly acetylated in the 0-2position. In separating the xylan from wood, the acetyl group is cleaved and as a result the pyrolysis spectrum for xylan shows little acetic acid (mlz 60). The distribution of products is similar to cellulose with a series analogous to the m/z 162,144,and 126 series in cellulose present at m / z 132,114,and 96-a difference of 30 m u due to the missing hydroxy methyl group in xylan. This shows that transglycosylation can occur without the primary hydroxyl group on the number 6 carbon, and one possible structure for mlz 132 would be 1,4-anhydroxylanopyranose.Previous has shown that the m / z 144 abundance for pure cellulose pyrolysis is largely due to ionization fragmentation and the species at mlz 114 may also be in part due to the ionization fragmentation of the mlz 132 species.
Pyrolysis of Biomass However, Ohnishi et aLsl in a study of xylan pyrolysis, found a product a t m / z 114 and identified it as 3hydroxy-2-penteno-l,5-lactone (XVII). This compound was also identified by Van der Kaaden et a1.52from the pyrolysis of amylose. Schulten et in field ionization mass spectrometry (FIMS) studies of xylan pyrolysis in vacuum, also found high abundances of m/z 132,114,and 96. The m / z 132 peak was about equal in height to the m/z 114 peak in FIMS studies, indicating that significant pyrolysis products a t m / z 114 are probably present since field ionization minimizes ionization fragmentation. In the pyrolysis of most biomass samples, m / z 114 is a significant peak, especially in grass species and hardwoods. The bagasse and birch pyrolysis spectra in Figure 3 show that m/z 114 is among the major peaks. The primary pyrolysis of lignins separated by a variety of methods has been previously reported.34 Among the common methods of separating lignin from wood, only milled-wood lignin (MWL) shows pyrolysis behavior that resembles the pyrolysis of lignin in whole biomass. Other methods of separating lignin, such as steam explosion, kraft, or organic solvent extraction, change the nature of the lignin and affect the pyrolysis product distribution. Even ball milling causes notable changes in the pyrolysis product distribution. In Figure 3, sweet gum MWL and whole sweet gum pyrolysis spectra are shown and there is a greater relative abundance of m / z 180 and 210 relative to the other lignin products (e.g., mlz 194) in the whole wood than in the milled-wood lignin spectrum. As noted for the carbohydrates, the lignin fraction of biomass also undergoes primary pyrolysis by structurally controlled depolymerization. The sweet gum MWL in Figure 3 shows the predominance of the precursor monomers of hardwood lignin a t m / z 180 (coniferyl alcohol (XXXIX)) and 210 (sinapyl alcohol (XLII)). These products not only form in high abundance, but also form early. The evolution profile of several major products from pine are shown in Figure 2. Coniferyl alcohol is the earliest pyrolysis product to form (the species a t m/z 226 is an extractive that is volatilized). The next product to form is derived from hemicellulose, m/z 114. This is followed by the cellulose derived species a t mlz 43. The last products to form are derived from lignin again, shown in Figure 2 by m / z 138 (methylguaiacol). Actually, lignin peaks are sequentially evolved over the time needed for complete devolatilization of the particle. The formation of the double bond in the alkyl side chain allows devolatilization with minimum need for transferable hydrogen. The other early predominant products also show the formation of a double bond in conjugation with the aromatic ring: isoeugenol (mlz 164),vanillin (mlz 152), and vinylguaiacol (mlz 150). The higher molecular weight series of peaks also show this mechanism and a possible structure for m/z 272 is an enol ether dimer34(XLIII). The whole sweet gum wood sample in Figure 3 shows that the generation of the precursor monomers is even more predominant in whole biomass than in the separated milled wood lignin. The other class of lignin primary products are the lower molecular weight primary products, such as methylguaiacol ( m / z 138),guaiacol (mlz 124),and catechol ( m / z 110). These products tend to reach their maximum evolution rate a t the end of pyrolysis as shown by m/z 138 in Figure 2. This is consistent with the observation in (51) Ohnishi, A.; Kato, K.; Takage, E. Carbohydr. Res. 1977,58,387. (52) Van der Kaaden, A.; Haverkamp, J.; Boon, J. J.; de Leeuw, J. J . Anal. Appl. Pyrolysis 1983, 5 , 199-220. (53) Schulten, H.-R.; Bahr, U.; Gortz, W. J . Anal. Appl. Pyrolysis 1981/1982,3, 229-241.
Energy & Fuels, Vol. 1, No. 2, 1987 131
thermogravimetric analysis that cellulose pyrolyzes over a relatively narrow temperature range while lignin pyrolyzes over a much wider range. The lignin from grass species has three monomers: coniferyl, sinapyl, and coumaryl alcohols. Coumaryl alcohol (m/z 150),which has no methoxy groups, is probably reponsible for mlz 150 in the bagasse sample in Figure 3. Vinylphenol (mlz 120) is a major low molecular weight lignin peak from grasses, which not only forms in high abundance from virtually every grass species but, unlike other low molecular weight lignin products, also forms early with an evolution profile similar to the lignin precursor monomers. One explanation for the late evolution of most low molecular weight lignin products is that during the early evolution of the alkenyl aromatic species (i.e., coniferyl alcohol), the bulk of the lignin is becoming more deficient in hydrogen. A portion of the bonds that are cleaved in the hydrogen-deficient matrix are likely to undergo condensation reactions, leading to a more refractory solid as pyrolysis progresses that can only undergo further devolatilization by hydrogen transfer to form stable aromatic species such as guaiacol. This means that lignin pyrolysis covers a wide range from the most labile to the most refractory species. At the latter stage of product evolution the methoxy groups are cleaved and there is the simultaneous evolution of dihydroxybenzene and methane.54 This is the only source of methane from the primary pyrolysis of biomass observed in this work. (The detection conditions used in the experiments reported here do not allow observation of methane in these spectra due to mass discrimination and low ionization sensitivity.) In summary, primary pyrolysis vapors are rather low in molecular weight, representing monomers and fragments of monomers of the biopolymers of biomass, no chemical interactions are observed among the organic components of biomass, mineral matter is suspected of altering the carbohydrate pathways substantially in wood in contrast to pure cellulose, and the patterns for the separate components of biomass are rather characteristic. Nature of Vapor Phase Cracking Products from Biomass Pyrolysis. When cracking of primary vapors is observed by increasing the temperature of the upper column of the reactor shown in Figure 1 beyond 500 "C, systematic changes are seen in the behavior of the primary pyrolysis products of wood and its components at residence times of the order of tenths of seconds. Examples of spectra observed for cellulose and lignin are shown in Figure 4 and for xylan (a representative hemicellulose) and wood in Figure 5. Tables I1 and I11 show prominent secondary cracking peaks that grow a t the expense of primaries. Many of these have been seen in liquids subjected to high cracking severity (see below ands5). One can distinguish a "primary oxygenates" zone from 400 "C (same spectrum as 500 "C) to about 700 O C , a "hydrocarbon* or secondary zone from 700 to 850 "C and a largely "aromatic" or tertiary zone above 850-1000 "C. A full unraveling of this complex chemistry will require the study of a number of pure primary pyrolysis products, since even a single compound such as coniferyl alcohol gives a dozen or so major secondary products.56 (54) Evans, R. J.; Soltys, M. N.; Milne, T. A. Annual Report, Oct 1, 1983 to Dec 30, 1984, SERI/PR-234-2701; Solar Energy Research Institute, Golden, CO. (55) Elliott, D. C. In Proceedings of the 2985 Biomass Thermochemical Conuersion Contractors' Meeting, PNL-SA-13571,CONF-8510167; NTIS: Springfield, VA, 1986; p 361-382. (56) Evans, R. J.; Milne, T. A. Proceedings of the 1986 Biomass Thermochemical Conversion Contractors' Meeting, PNL-SA-13571, CONF-8510167;NTIS: Springfield, VA, 1986; p 57-79.
Evans and Milne
132 Energy &Fuels, Vol. 1, No. 2, 1987 100
28
78
1
h
c .v) C
Cellulose 95O0c
Sweet gum lignin 9 5 0 " ~
Cellulose 7OOOc
Sweet gum lignin 7OOOc
Q)
c
.-C
181
I Sweet gum lignin
Cellulose 500" c
500"c 151
loo[
m/z
137
~
I
m/z
Figure 4. Vapor phase cracking of the primary products derived from cellulose and milled-wood lignin from sweetgum wood at 500, 700, and 950 O C and residence times of about 750 ms. The primary pyrolysis was like that used in Figure 3 except the gas temperature of the lower reactor was increased to match the secondary zone. This change in primary pyrolysis surrounding gas temperature had little effect on the composition of the primary products; see Experimental Section for details. Table 11. Prominent Intermediate Ion Peaks in the Secondary Cracking Regime of Wood ion, m / z formula Dossible compd methane 16 CH4 ethene 28 CZH, ethane 30 CZH6 40 C3H4 ProPYne propene 42 C3H6 carbon dioxide 44 COZ butadienes, butyne 54 C4H6 butenes 56 C4H8 cyclopentadiene 66 C5H6 toluene C7H8 92 phenol 94 C6H60 styrene 104 C8H8 xylene 106 C8HIO cresol 108 C7H8O dihydroxybenzene 110 C6H602 benzofuran(?) 118 C8H60 vinylphenol 120 C8H80 benzodioxole (?) 122 C7H60Z dihydroxytoluene, guaiacol 124 C&Oz 130 methylbenzofuran (?) 132 C9H80 methylbenzodioxole (?) 136 CBH8OZ 142 144 vinylbenzodioxole (?) 148 CSH8OZ 160 168 182
The s p e c t r a at 500 "C for the four materials shown in Figure 4 and 5 are dominated b y peaks that are all due to primary products; however, there is some decrease i n the
Table 111. Prominent Growth Ion Peaks in the Tertiary Cracking Regime of Wood ion, m / t formula possible compd 16 CH, methane 18 water 26 acetylene 28 carbon monoxide 44 carbon dioxide 52 vinylacetylene (?) cyclopentadiene 66 78 benzene to1uene 92 94 phenol 104 styrene 116 indene 128 naphthalene 152 acenaphthylene 166 fluorene, benzindenes 178 anthracene, phenanthrene 202 pyrene, fluoranthene, benzacenaphthylene 216 methylpyrene 226 benzo[ghi]fluoranthene 228 chrysene, benz[a]anthracene, triphenylene, benzo[clphenanthrene 240 methylbenzo[ghi]fluoranthene 252 benzofluoranthenes, benzo[ae]pyrene, perylene 276 anthanthrene et al. relative abundance of the heavier species. No major change i n composition occurs until approximately 550 "C where the first species to crack are the alkenyl aromatics, such as coniferyl alcohol. A significant decrease i n mlz 180 and 210 is already observed at 500 "C for sweet g u m MWL i n Figure 4.
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987 133 100
Xylan 950"
Pine 950"
128
'
is','
292
178
152
ins
I
iA
x
'
2A
'
2;s
" 2 i 0,'
Xylan 700"
c .-
ln
0
"'3b
C
e,
x
Pine 700'
c .-
ln C
e,
c
c
.-c
> .-c m -
.-c e, > .c m -
a
a
e,
a
e,
'"r L
137
Xylan 500"
Pine 500'
272
0
m/z
m/z Figure 5. Vapor phase cracking of the primary products derived from xylan and pine wood at 500, 700, and 950 "C and residence times of about 750 ms. The primary pyrolysis was like that used in Figure 3 except the gas temperature of the lower reactor was increased to match the secondary zone. This change in primary pyrolysis surrounding gas temperature had little effect on the composition of
the primary products; see Experimental Section for details.
Cellulose products are essentially unchanged a t the lowest temperature when that spectrum is compared to the primary spectrum in Figure 4. Levoglucosan is known to be thermally labile so the lack of change in product distribution between the primary slate (750-ms gas phase residence time in Figure 4) and the 750-ms slate at 500 "C (in Figure 5 ) is surprising. Shafizadeh and Lai5' studied the thermal degradation of levoglucosan at 600 "C but with an extreme residence time of 8 min. They found cracking to carbon dioxide, carbon monoxide, and carbonyl compounds. No increase in the relative abundance in m/z 43 is observed due to increased gas phase residence time, indicating only minimal carbonyl compound formation. This is surprising since retroaldolization is a likely pathway from the anhydropyranose sugar. The spectra for xylan is also largely unchanged by the increase in gas phase residence time from approximately 75 to 750 ms. Within the secondary cracking zone, sliown in Figures 4 and 5 by the spectra at 700 "C and 750 ms, major changes for all products are observed. Cellulose is dominated by C 0 2 and CO with low molecular weight alkenes such as ethylene, propylene, and butene. Aromatic species have also formed such as furan, benzene, toluene, and phenol. The levoglucosan has been completely decomposed. The same secondary product slate is also observed from xylan even though it is a five-carbon sugar and has a primary slate diffferent from that of cellulose. The xylan primary producta are essentially 30 amu less than those from cellulose (96,114, and 132 vs. 126, 144, and 162, respectively). (57) Shafizadeh, F.; Lai, Y. Z. J. Org. Chem. 1972, 37, 278-284.
These products are not likely to be more aromatic in nature than the primary products from cellulose. This indicates that benzene, toluene and phenol from carbohydrates are probably due to gas phase polymerization of unsaturated species such as propylene, butadiene, and butene. The lignin secondary products have appreciable yields of low molecular weight aromatics at 700 "C, however, and this is undoubtedly due to cracking of the higher molecular weight methoxy phenols (e.g., m / z 150, 164, 180, etc.), which are no longer present in the lignin and pine wood spectra at 700 "C. The peaks at m/z 94,110, and 124 are probably due to the initial phenolic secondary products from lignin cracking (the peak at m/z 124 is probably due to both methylcatechol and guaiacol, which is a secondary as well as a primary product). The peak at m / z 136 is a major intermediate product, and although this may also be a phenol, a possible structure is methylbenzodioxole (XLVI), which could be formed by the intramolecular condensation of the methoxyl and hydroxyl groups. A homologous series is possible with benzodioxole at m/z 122 serving as the basis for the peaks at m / z 136 (methyl-), 148 (vinyl-), 162 (l-propenyl-),and 178 (l-propen-3-al). Past on the cracking of vanillin (mlz 152) has shown a major peak at m l z 150, and the cracking of isoeugenol (mlz 164) gave a peak at mlz 162 in addition to other products. The cracking of the MWL is very complicated, especially at the higher masses, in contrast to the other three samples, although the pine wood does have some high molecular weight peaks. Despite these high molecular weight species there is no evidence of condensed aromatics, such as polynuclear aromatics, in this inter-
Evans and Milne
134 Energy &Fuels, Vol. 1, No. 2, 1987
mediate zone. Although it is impossible to distinguish isomers, the polynuclear aromatics are thought not to be present in the secondary regime (700 "C epectrum) since plots of intensity vs temperature show these species increasing from low values a t 700 "C to very high values at 950 "C. Therefore, it is not likely that they are present at 700 "C and simply survive at the higher temperature relative to the other species present a t 700 "C. The tertiary products are shown a t 950 "C and 750-ms residence time. The product distribution from the carbohydrates is still dominated by CO (with ethylene also contributing to m/z 28) and COz, but the formation of benzene and the heavier aromatics such as naphthalene ( m / z 128) and anthracene (mlz 178) have increased and those aromatics containing oxygen, such as furan and phenol, have been cracked. The olefins are decreasing within this zone and since the higher aromatics grow in absolute terms from zero, they are probably due to the continued polymerization of the unsaturated species. The lignin and wood samples have a higher proportion of high molecular weight aromatics indicating the importance of the light aromatics that are directly formed from the lignin primary materials. The building from the lignin aromatics is not a necessary step for polynuclear aromatics, however, since the same types of aromatic products are obtained from cellulose and xylan where no aromatic structures exist as primary products. The growth of the polynuclear aromatics occurs beyond the temperature where the primary products survive, which indicates that the condensed aromatics are derived from the polymerization of low molecular weight hydrocarbons along with benzene and toluene formed directly from the gas phase cracking of lignin primary products. The chemistry of the cracking of biomass in this kinetic regime is dominated by hydrocarbon cracking reactions. The generation of olefins has been a major goal of fast-pyrolysis research.23 Early work by Antalg on the pyrolysis of cellulose-derived primary products showed that methane and ethylene were produced by a common path and that all gas-producing pathways generated roughly equal amounts of CO. However, as mentioned previously, the cellulose pyrolysis in most biomass does not occur via the transglycosylation mechanism but via the glycosidic cleavage mechanism, which is catalyzed by the alkali-metal material present in the biomass matrix. This primary product slate has secondary cracking products different from those of pure cellulose. The yields of benzene, phenol, and cyclopentadiene are significantly higher from the alkali-metal-treated cellulose,Mand the rate of formation of ethylene and other olefins may be sufficiently different from the rate of CO formation that significant improvement in ethylene yield could be obtained by pretreatment with alkali-metal salts. Multivariate Analysis of Vapor Phase Transformations. As mentioned above, the recent work by Windig et a1.41358959in multivariate analysis of pyrolysis mass spectrometric data provides a means of simplifying the data and observing trends. The interested reader should consult the work of Windig et al. for an in-depth description of this technique. In brief, data reduction is accomplished by finding correlated masses that can be expressed by a new single variable, the factor. In a typical pyrolysis data set, greater than 95% of the variation in the 100 most important masses in the data set is contained in ~~
(58) Windig, W.; Kistemaker, P. G.; Haverkamp, J. J. Anal. Appl. Pyrolysis, 1981/1982, 3, 199-212. (59) Windig, W.; Haverkamp, J.; Kistemaker, P. G. Anal. Chem. 1983, 55,81-88.
only 5-10 factors. The frst factor is the linear combination of masses that accounta for more of the variance in the data than any other combination of variables. The subsequent factors are similarly extracted on the basis of the residual variance after the effect of the previous factors has been removed from the data. If the masses are completely uncorrelated, then there would be as many factors as masses and no data reduction would occur. However, as shown in the primary and secondary pyrolysis results, the masses are highly correlated so significant data reduction is possible. This data reduction makes possible graphical display of the data that not only shows trends but also can provide chemical insight into the transformations. Each mass has a correlation coefficient, "the loading", aij,with each of the factors. The masses with high loadings on a particular factor are correlated, and that factor represents that group of masses and is quantified for each sample by the factor score, Fj. The factor score for a sample is a linear combination of the masses based on the loading of a mass for a factor and the intensity of that mass, Z, for the sample:
By use of the factor loadings for the masses and factor scores for the samples, factor analysis can reflect the presence of chemical components or trends in the data set. After the preparation of the correlation matrix between masses and the extraction of the initial factors, a third operation that is generally performed is the rotation of the mathematically derived factors to coincide with real chemical components. Certain real chemical components will have high loading on more than one initial factor. Windig and Meuzelaar41developed variance diagrams that graphically show which linear combination of factors best describe the component of interest. A description of the methodology is beyond the scope of this work and again the interested reader is directed to the work of Windig et a1.41,58959
The secondary cracking of pine wood pyrolysis products was studied a t three residence times from 500 to 1000 "C. Factor analysis was performed on the 100 masses in the data set with the most variance by using the SIGMA program developed by Windig et a1.60 Ten factors were extracted from the data, which together accounted for 98.2% of the original variance. The plot of factor score 1vs factor score 2 is shown in Figure 6a. The factor score plot shows the well-behaved transition of sample composition as a function of temperature and residence time. The trend shown graphically by factors 1and 2, due to variation in vapor phase residence time and temperature, can be given chemical meaning by studying the major masses which contribute to the two factor scores. In Figure 6b is shown the variance diagram described above that shows the location of the major components expressed as degrees from factor 1. A major maximum is seen at 340°, and from a comparison of the variance diagram to the factor score plot, it can be seen that this location corresponds to the temperatures between 500 and 600 "C. The representation of the loading as a factor spectrum in Figure 7a for this linear combination of factors 1and 2 (340"rotation) shows that this represents the primary products, and it is logical that the low-temperature samples will be located in this (60) Windig, W.; Chakravarty, T.; Richards, J. M.; Nguyen, V. T.; Dedes, A.; Meuzelaar, H. L. C. "SIGMA, an IBM 9OOO computer software system for interactive, graphics-oriented multivariate analysis"; on loan from the Biomaterials Profiling Center, the University of Utah,Salt Lake City, UT, 1986.
Pyrolysis of Biomass a)
Energy & Fuels, Vol. 1, No. 2, 1987 135
-
43
150ms
a) 340" 0 750 ms
kooo1t
500 Temperature
("C)
F2
110
-2 .-
0.8' 1
5
I
94
1
m 2 O.S
911 44
I
28
I
I/
'I
b) 90"
1
j
I
I'
128
270"
Figure 6. Fador analysis of mass spectrometric data for the effect of gas phase temperature and residence time on product distribution: (a) plot of factor score 1vs. factor score 2; (b) variance diagram showing maxima at 340,90,and 210' rotation where the
main chemical component vectors lie for primary, secondary, and tertiary products, respectively. The maxima indicate the linear combinations of Fland F2 that best describe the chemical components. The factor spectra for these components are shown in Figure 7.
region. This component axis a t 340° in Figure 6b can be used to quantitate the cracking of the primary products as a function of time and temperature. The variance diagram shows three intermediate maxima at 40,90, and 120° rotation (the maximum a t 90° corresponds to F2). Those three maxima correspond to degrees of secondary cracking. Only the maximum at 90° will be discussed here with the maximum at 40' displaying less cracking severity and the maximum at 120° displaying more cracking severity (i.e., more phenols a t 120O). The factor spectra at 90° (Figure 7b) shows that this factor represents the secondary products with high abundances of low molecular weight hydrocarbons (i.e. ethane, mlz 30; propylene, m / z 42; butadiene, m / z 54), the phenols (m/z 94, phenol; m / z 108, resol; mlz 110,catechol; m / z 120, vinylphenol) as well as other products which have not been identified yet. The third maximum in the variance diagram is a t 210° rotation, and the factor scale plot shows that this is the region of high temperature. The factor spectrum for 210' rotation shows that this group contains
0
m/z
Figure 7. Factor spectra of the component axes (as located by the maxima of the variance diagram in Figure 6)that explain the main chemical trends in vapor phase pyrolysis of wood-derived volatiles: (a) the factor spectrum at 340' rotation showing the primary products; (b) the factor spectrum at 90' showing the secondary products; (c) the factor spectrum at 210' showing the tertiary products.
the tertiary products CO and C 0 2 ,benzene, and the condensed aromatics. These two variables can thus be used to describe the major transitions in biomass vapor phase chemistry and graphically show the relative importance of temperature and residence time within the major cracking regions. Summary of Pyrolysis Pathways. We summarize our observations of the pyrolysis pathways for whole biomass (and its components) in the simple diagram shown in Figure 8. The intent is to show the origin of the major products in both high-pressure and low-pressure pyrolysis,
Evans and Milne
136 Energy &Fuels, Vol. 1, No. 2, 1987 Primary processes
Secondary processes
Primary
Vapor
Light hydrocarbons, aromatics and oxygenates
phase”
I
Primary liquids
\ &
Condensed 011s (phenols, aromatics)
I Solid phase
I1
Biomass’
Olefins, Aromatics CO. COz, H2, HzC
High pressure
1
Liquid phase”
c
Tertiary processes
*
1 soot
Charcoal
Pyrolysis severity
__t
‘This box is meant to include solid biomass but also its plastic or rubbery forms prior to major rupture of covalent bonds to produce low molecular weight vapors or liquids
”Only first-formed products are shown All of these products could be condensed to liquids and solids
Figure 8. Pyrolysis pathways that are consistent with observation reported in this paper.
with an emphasis on the latter. The solid products can be distinguished by their origins: charcoal retaining the morphology of the original lignocellulosic;coke arising from continued thermolysis after the deposition of liquids and organic vapors; soot from homogeneous nucleation of high-temperature decomposition products of hydrocarbons from the vapor phase. The direct production of liquids is postulated to occur mainly at pressures above atmospheric. Diebold has shown that under direct-contact, fast-pyrolysis conditions, wood exhibits many properties of a “molten plastic state”.23 IrvineG1has shown that wood has a glass transition temperature, a t a low temperature of 60-90 “C, that is highly dependent on the amount of water present as well as derived essentially from the lignin component. According to Irvine, this glass transition is associated with the onset of “rubbery flow” caused by slippage of the biopolymers. Under high-pressure conditions, the direct formation of a liquid product slate due to the pyrolysis of the “plastic” wood is postulated to occur. The primary, true liquids formed in the rapid pyrolysis of biomass under pressure (but in the absence of catalysts or reactive atmospheres) appear to have the same composition as the vapor obtained directly from low-pressure pyrolysis.62 The further, high-pressure conversion of the primary liquid is practiced under a great variety of conditions, often with catalysts and added H2 or CO. The discussion of the condensed oils is beyond the range of this article, but has recently been reviewed by Beckman and Elliott.63 Pyrolysis mass spectra of several condensed liquids are given in Milne et a1.64 The upper, vapor-phase, pathway in Figure 8 is the (61) Irvine, M. G. TAPPI J. 1984,67, 118. (62)Evans, R.J.; Milne, T. A,, submitted for publication in Energy Fuels. (63) Beckman, D.; Elliott, D. C. Can. J. Chem. Eng. 1985,63, 99. (64)Milne, T.A.; Evans, R. J.; Soltys, M. N. In Energy from Biomass and Wastes VIII; Institute of Gas Technology: Chicago, IL, 1984;
1371-1374.
major one elucidated by our work. At atmospheric pressure or below it is not clear whether a liquid phase exists, after breaking of the main polymer covalent bonds, prior to volatilization of the main components of wood. As noted lignin is known to soften a t rather low temperatures, and the charcoal, though retaining structural features of the wood, does shrink, which could indicate a plastic state where pyrolysis products pass directly into a liquid state before devolatilization. Our experimental system does not provide any insight into the condensedphase transitions that occur in pyrolysis, and this interesting hypothesis should be further explored by other experimental techniques. The direct formation of gaseous species from primary pyrolysis reactions, the prompt gases, as reported by many workersB are primarily COz,H20,and CO. These are largely associated with the char forming reaction^.^ The primary vapors are very similar under high- and low-pressure conditions. In fact, our primary spectra from fractions of a gram at one atmosphere are qualitatively similar to Curie-point and field ionization pyrolysis results from micrograms volatilized in hard v a c u ~ m . Figure ~ ~ , ~3~illustrates the range of spectra observed for the primary vapors. The sequential transformation of the primary products in the vapor phase is shown in Figure 8 as going through three stages. Between 500 and 600 OC slight cracking reactions occur (on a time scale of about one second) before substantial conversion to permanent gases occurs. (This stage is not shown in Figure 4 and 5 but is the subject of current research activity. The factor analysis results in Figures 6 and 7 indicate potential combinations of temperature and residence time to achieve slight cracking intermediate between the product slates shown in Figure 7a,b.) The higher molecular weight lignin products are cracked to lighter aromatics and oxygenates. This stage may be of practical interest in conversion techniques since several desirable transformations occur without substantial loss of liquid yield: the cracking of coniferyl alcohol, which likely polymerizes in the condensed, acidic phase, hence
Pyrolysis of Biomass
raising the molecular weight of the products; organic acid decarboxylation, increasing the pH from the initial value of approximately 2. The second stage is the formation of secondary products characterized by CO, light olefins, and the formation of aromatics, even from the carbohydrates. The spectra a t 700 "C in Figures 4 and 5 demonstrate the products in this class. This regime is of interest since high-value olefins and light aromatics are a desirable product slate and have been studied in depth.12 The third stage leads to the tertiary products characterized by the polynuclear aromatics as shown in Figure 7c and listed in Table 111. These products generally form only in high-temperature conversion processes such as gasification and combustion and generally in low yield. These classes of products are logical generalizations from our mass spectrometric studies and provide a means of relating this work to practical methods of thermochemical conversion, such as primary pyrolysis liquid production, catalytic conversion of the primary products, secondary cracking of the primary products to hydrocarbon gases, the nature of the residual tars produced in gasification, and source of the types of products found in emissions from wood combustion. These applied topics will be covered in the next paper in this which focuses on this classification scheme and MBMS studies of applied thermochemical conversion systems. Acknowledgment. ,The authors gratefully acknowledge the support of the DOE Biofuels program under Simon Friedrich and Gary Schiefelbein and the technical monitoring of Donald Stevens. Collaborations and discussions with James Diebold, Tom Reed, Helena Chum, William Peters, Douglas Elliott, and Michael Anta1 were particularly valuable. Crucial experimental and design help with the MBMS was provided by Michael Soltys. The SIGMA software for multivariate analysis was provided by Willem Windig and Henk Meuzelaar of the Biomaterials Profiling
Energy &Fuels, Vol. 1, No. 2, 1987 137
Center, University of Utah. Registry No. I, 110-00-9; 11, 534-22-5; 111, 14320-37-7; IV, 120-92-3;V, 497-23-4; VI, 96-48-0; VII, 2453-00-1; VIII, 98-01-1; IX, 1120-73-6;X, 98-00-0; XI, 591-12-8; XII, 25414-24-8; XIII, 108-29-2; XIV, 620-02-0; XV, 80-71-7; XVI, 4041-09-2; XVII, 55100-07-7; XVIII, 2628-17-3; XIX, 90-05-1; XX, 67-47-0; XXI, 118-71-8; XXII, 93-51-6; XXIII, 68732-99-0; XXIV, 7786-61-0; XXV, 3690-05-9; XXVI, 2785-89-9; XXVII, 121-33-5; XXVIII, 91-10-1; XXVIX, 498-07-7; XXX, 97-54-1; XXXI, 498-02-2; XxXII,663805-7; XxXIII,458-36-6; XXXIV,458-35-5;XXXV, 28343-22-8; XXXVI, 134-96-3;XXXVII,14059-92-8; XXXVIII, 6635-22-9; XXXIX, 1135-24-6; XL, 2478-38-8; XLI, 4206-58-0; XLII, 537-33-7; XLN, 106544-450;XLV, 492-62-6; MeH, 74-82-8; CO, 630-08-0; C2H4, 74-85-1; MeOH, 67-56-1; OCO, 124-38-9; OCHMe, 75-07-0; HCOOH, 64-186; MeCOMe, 67-64-1; H02CMe, 64-19-7; HOCH2CH0, 141-46-8;MeOCHO, 107-31-3;HOCH2CH,CHO, 2134-29-4;EtCOMe, 78-93-3; CH,=CHCOOH, 79-10-7; HOCH2COMe,116-09-6; HO,CEt, 79-09-4; CH3COOCH3,79-20-9; HOCH,COOH, 79-14-1; MeCOCOMe, 431-03-8; MeCH= CHCOOH, 372465-0; HO,CEtMe, 107-92-6;MeCH2COCH20H, 5077-67-8; PhOH, 108-95-2; MeCOCOEt, 600-14-6; CH3(CH2),COOH, 109-52-4;o-CH~C&OH, 95-487; m-CH3CsH,OH, 108-39-4; P-CH&H~OH, 106-44-5;CeH,(OH)2, 12385-08-9;CH3(CHz),COzH, 142-62-1; CH&O&H2(CO)CH3,592-20-1; C G H ~ ( C H ~ ) ~ O H , 1300-71-6;C ~ H I ( C H ~ C H [ ~ ) O 25429-37-2; H, C&&,CO2H, 65-85-0; C&, 74-84-0; C3H4,74-99-7;C3H6,115-07-1; CzHz, 74-86-2; C&3, 71-43-2; K2C03,584-08-7; K, 7440-09-7; 1-hydroxy-2-propanone acetate, 106544-46-1; 1-hydroxy-2-butanone acetate, 1575-57-1; eugenol, 97-53-0; butadiene, 106-99-0;butyne, 25339-57-5;butene, 25167-67-3; cyclopentadiene, 542-92-7;toluene, 108-88-3; styrene, 100-42-5;xylene, 1330-20-7; indene, 95-13-6; naphthalene, 91-20-3; acenaphthylene, 20896-8; fluorene, 86-73-7; anthracene, 120-12-7; phenanthrene, 85-01-8; pyrene, 129-00-0; fluoranthene, 206-44-0; benzacenaphthylene, 76774-50-0; methylpyrene, 27577-90-8; benzo[ghi]fluoranthene, 203-12-3; chrysene, 218-01-9; benz[a]anthracene, 56-55-3; triphenylene, 217-59-4; be"[ clphenanthrene, 195-19-7;methylbenzo[ghi]fluoranthene,51001-44-6; benzo[ae]pyrene, 73467-76-2; perylene, 198-55-0; anthanthrene, 191.26-4; cellulose, 9004-34-6; lignin, 9005-53-2; hemicellulose, 9034-32-6; xylan, 9014-63-5.
