DBB6ISJ1IB1S
AN AMERICAN CHEMICAL SOCIETY JOURNAL
VOLUME I, NUMBER 2
MARCH/APRIL 1987
©Copyright 1987 by the American Chemical Society
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Articles
Molecular Characterization of the Pyrolysis of Biomass.
Fundamentals
1.
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 sum 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, C02, and H20. These
steps are illustrated and discussed. Consistent with these observations, a relatively simple pyrolysis
reaction scheme is proposed.
Introduction
cialists meetings covered major aspects of pyrolysis12,13 so
attempt will be made in this paper to summarize the
whole field. (Excluded from this discussion are the complex, condensed-phase, 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.)
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
wood.1-6 More 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,6-11 and two spe-
no
(8) Shafizadeh, F. J. Anal. Appl. Pyrolysis 1982, 3, 283-305.
(9) Antal, M. J. Jr. Adv. Solar Energy 1982, 1, 61-112.
(10) Antal, M. J. Jr. Adv. Solar Energy 1985, 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,
(1) Roberts, A. F. Combust. Flame 1970, 14, 261.
(2) Beall, F. C.; Eickner, H. W. Res. Pap. FPL—For. Prod. Lab. (U.S.)
1970, FPL 130.
(3) Shafizadeh, F. Adv. Carbohydr. Chem. 1968, 23, 419-474.
(4) Gooe, A. W. In Wood Chemistry, 2nd ed.; Wise, L. E., Jahn, E. C.,
Eds.; 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) Milne, T. A. In A Survey of Biomass Gasification; Reed, T. B.; Ed.;
Noyes Data Corp.: Park Ridge, NJ, 1981; Vol. 2, Chapter 5.
(7) Soltes, E. J.; Elder, T. J. In Organic Chemicals from Biomass;
Goldstein, I. S., Ed.; CRC; Boca Raton, FL; 1981 pp 64-99.
0887-0624/87/2501-0123$01.50/0
CO, 1980.
(13) Overend, R. P., Milne, T. A., Mudge, L. K., Eds. Fundamentals
of Thermochemical Biomass Conversion; Elsevier Applied Science: New
York, 1985.
(14) Molton, P. M.; Demmitt, T. F. Battelle Pac. Northwest Lab.,
[Rep.] BNWL 1977, BNWL-2297.
(15) Chornet, E.; Overend, R. P. Bioenergy 84 1985, 3, 276-296.
(16) Chornet, E.; Overend, R. P. In Fundamentals of Thermochemical
Biomass Conversion; 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 1985, 7, 99-123.
©
1987 American Chemical Society
124
Energy & Fuels, Vol. 1, No. 2, 1987
Evans and Milne
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 techniques18 or have collected final gaseous,
liquid and solid products for analysis and to close mass
balances.19 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 observed.9,22"24 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.26 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 Biomaterials-, 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, PNLSA-13571, CONF-8510167; NTIS: Springfield, VA, 1986; p 15.
(25) Milne, T. A.; Green, F. T. J. Chem. Phys. 1972, 56, 3007.
Figure 1. Schematic of pyrolysis vapor generator coupled to a
molecular-beam mass spectrometer sampling system.
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 in several papers and reviews.26-31 The main
are
features for our sampling applications are as follows: collisions
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 “fingerprint" 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 1 for 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
are
have been published.26,32-34
(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, 76,
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) Stearns, C. A.; Kohl, F. J.; Fryburg, C. G.; Miller, R. A. NBS Spec.
Publ. (U.S.) 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
2.
Temperature profiles in the upper, vapor phase cracking region
measured by using doubly shielded thermocouples to establish rough constancy to ±10 °C. This was felt to be more than
adequate for the qualitative deductions reported here. Residence
times were simply calculated from bulk flow at 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 Teknivent35 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 batch 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 at 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 quadrupole,36 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 weight.37 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., CH4, H20).
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 sis GC, GC-MS,
FTIR, condensation of products followed by chemical analysis,
etc. Its outstanding advantages are the rapidity of data acquiwere
5
19
15
29
25
39
35
49
45
59
55
49
Time (s)
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
(s); (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.9 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 (>103 °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.27 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 to
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.
(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.
(37) Greene, F. T.; Brewer, J.; Milne, T. A. J. Chem. Phys. 1964, 40,
1488-1495.
126
Evans and Milne
Energy & Fuels, Vol. 1, No. 2, 1987
sition, the fact that all volatile species can be directly sampled
from their reactive environment, and the nearly universal detection
capability of mass spectrometry. Because no prior separation is
made, as in GC-MS, isomers can seldom be distinguished and
severe fragmentation can obscure parent molecules. Fragmentation can be used to distinguish compounds with the same
nominal molecular ion. Ethylene, for example, has a fragment
ion at m/z 27, which can be used to determine the relative contribution to m/z 28 of ethylene and CO. These disadvantages
are in part offset by the ability to observe 100% of pyrolysis vapors
that by most techniques are often only 30-70% chromatographable when the condensed vapors are revaporized.38-40
Finally, mass spectral “fingerprints,” particularly if fragmentation is minimized, are more intuitively interpretable than gas
chromatograms, since the displayed axis is molecular weight and
resolution is quantized and roughly constant. In general the mass
spectra, vis-a-vis gas chromatograms, are simplified by the
overlapping of parent ions of isomers, but are complicated by
virtue of fragment ions. Ultimately it is probably not affordable,
or desirable, to attempt to routinely quantify the hundreds of
minor species known to be involved in pyrolysis. What will be
more important is the behavior of major products or functional
classes of compounds, which affect product properties and define
mechanistic pathways. The rapidly emerging techniques of
multivariate statistical analysis, such as factor analysis,41 combined
with the ready acquisition of pyrolysis “fingerprints" may be the
preferred strategy for exploratory research.
Results and Discussion
A number of reviewers of biomass pyrolysis have postulated more or less complex reaction schemes to account
for slow and fast pyrolysis and primary and secondary gas
phase pathways. These range from simple schemes3,9,10,44-46
to complex, multipath schemes.7,10,12
It has been a major goal of our work, and of this paper,
to add molecular detail to the substances variously labeled
as “volatiles”, “monomers”, “tar”, “anhydrosugars”,
“primary tars”, “low molecular weight liquids”,
"oxygenates”, “transient oxygenated fragments”,
“secondary tar”, “vapor-phase-derived tar”, “refractory
tars”, “refractory condensibles”, “secondary gases”, and
“pitch” and the pathways for their formation. One can
appreciate the need for a standardizations of nomenclature
for pyrolysis products. The simple use of “oil” and “tar”
is ambiguous for biomass-derived oxygenates, however, due
to the traditional use of those terms in the fossil literature.
Nature of Primary Products from Biomass Pyrolysis. The number of chemical species that have been
reported from wood pyrolysis and wood distillation is
enormous.
As an example Goos4 lists 231 compounds that
have been found in the liquid products from the destructive distillation of wood. Soltes and Elder7 in a recent
review, tabulate about 230 compounds identified in
“pyroligneous acid” and “tar” products from softwood and
hardwood pyrolysis. Part of the reason for these large
numbers is the variety of conditions under which wood
(38) Elliott, D. C. Final Report on IEA Co-Operative Project Dl,
Biomass Liquefaction Test Facility Project; Pacific Northwest Laboratoires: Richland, WA, 1983; Vol. 4.
(39) Beaumont, O. Wood Fiber Sci. 1985, 17, 228-239.
(40) Menard, H.; Belanger, D.; Chauvette, G.; Gaboury, A.; Khorami,
J.; Grise, N.; Martel, A.; Potvin, E.; Roy, C.; Langlois, R. Fifth Canadian
Bioenergy R&D Seminar; Hasnain, S., Ed.; Elsevier Applied Science:
New York, 1985; pp 418-439.
(41) Windig, W.; Meuzelaar, H. L. C. Anal. Chem. 1984,56, 2297-2303.
(42) Kilzer, F. J.; Broido, A. Pyrodynamics 1965, 2, 151-163.
(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. Dev. 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. Holzforschung 1983, 37, 267-269.
pyrolysis takes place and the varieties of biomass. Thus
various mixtures of primary and secondary organics are
obtained, depending on the thermal history and environment of the pyrolysis.
Mass spectra of “primary” pyrolysis products from a
number of samples of whole biomass and its major components are shown in Figure 3 and a listing of inferred
chemical species is given in Table I. By “primary spectra”
we mean observed spectra that are substantially free of
secondary gas phase cracking products as inferred from
the secondary cracking studies discussed below. Cracking
of primary products within the pyrolyzing particle can also
occur during their escape to the surface. Over the dimensions of particles typically pyrolyzed (1-6 mm narrowest dimension) only small changes in relative mass
spectral peak heights occur (less than a factor of 1.5 for
almost all peak ratios). Such changes have no effect on
the qualitative discussion to follow. For large particles,
of industrial interest, internal cracking would become important and should be quantified.
In our tabulation in Table I, we have restricted the list
to what we believe are primary pyrolysis products from
wood and its separated components, as revealed by parent,
or prominent fragment ions from fast-pyrolysis mass
spectra such as shown in Figure 3, and from recent analyses
of primary products from collected pyrolysis oils.38,39,40,46
Many confirming reports of these species exist in the literature7,14,46,47 No species are observed from wood that
appear to derive from a chemical interaction of the organic
components, though proportions may change due to matrix
effects and inorganic interactions. This is in agreement
with Goos’ observations.4 The references in Table I cite
independent, chemical confirmation of the compounds
listed.
Almost all of the even-mass ions observed in our mass
spectra can be related to observed chemical species, though
ambiguities are inherent in our mass spectra due to fragmentation, as the case of levoglucosan typifies.33 A number
of odd-mass ions (due to fragmentation since nitrogen is
virtually absent in wood) are prominent and are useful for
pattern recognition correlations. Of special note are m/z
43+ (CH30+), a common fragment from aldehydes, ketones,
and other carbonyl compounds, and m/z 57 and 73, which
are common
fragment ions of carbohydrates like levoglucosan.
Through spectra in this paper are shown only to 300
amu, we have carried out mass spectral scans to 1400 amu.
With the quadrupole adjusted to greatly over represent
high masses (as judged by the standard fragmentation
pattern of perfluorotributylamine) levoglucosan oligomers
up to the octamer and lignin polymers beyond the trimer
were seen from cellulose and lignin, respectively. On the
basis of calibrations at lower mass and comparisons of our
spectra for levoglucosan and lignin with field ionization
results,26,33 the higher molecular weight species are estimated to constitute less than a few percent of the vapor.
The main processes leading to the primary pyrolysis
spectra for carbohydrates and lignin have been extensively
discussed in two specialized reports48,34 as has the special
problem of levoglucosan.33 A synopsis of these mechanisms
follows.
Primary Pyrolysis Pathways. There have been two
main theories of cellulose pyrolysis: free radical and
heterolytic depolymerization. Both of these theories have
(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
Energy & Fuels, Vol. 1, No. 2, 1987
127
60
Cellulose
h
73
57
i
9B
144
85
43
.
25
162
126
L iliil
.a ................
ijL
50
|«aiin.
75
Jl
,imuMi.
160
-lil. Jl.
.ji.iiiii
125
,1
,uil
150
1
-i.....
175
-
—
-
-
.
200
225
250
T
-
275
^
300
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
°C helium, giving an estimated heating rate of 30-50 °C/s. The particle size ranged from 50 to 250 /um. 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 Antal10 and Evans48 and are only briefly summarized here. A free-radical mechanism has been proposed
by Kislitzyn et al.49 on 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
%. Shafizadeh8 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) Kislitsyn, 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 depth48 and
favor a concerted displacement reaction that is similar to
Shafizadeh’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 at 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%.8 Work with chemical ionization33 has shown that a major part of m/z 57, 60, 73, and
98 ions are also El 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
128
Evans and Milne
Energy & Fuels, Vol. 1, No. 2, 1987
Table I. Major Ions in Mass Spectra of Wood and Compounds Reported in Primary Oils and Gases
ref to
ion,
m/z
formula
structure
ch4
h2o
MeH
HOH
CO
CO
c2h4
ch2=ch2
likely product
32
ch4o
MeOH
44
C02
OCO
c2h4o
ch2o2
c3h6o
OCHMe
HCOOH
MeCOMe
H02CMe
HOCH2CHO
MeOCHO
methane
water
carbon monoxide
ethene
methanol
carbon dioxide
acetaldehyde
formic acid
2-propanone (acetone)
acetic acid
hydroxyacetaldehyde (glycoaldehyde)
methyl formate
I
fur an
EtCOMe
2-butanone
acrylic acid
l-hydroxy-2-propanone (acetol)
propanoic acid
hydroxypropanal
methyl acetate
glycolic acid
2-methylfuran
cyclopentenone
cyclopentanone
2(57f)-furanone
2,3-butanedione
crotonic acid
butyrolactone
butanoic acid
l-hydroxy-2-butanone
phenol
dimethylcyclopentene
16
18
28
46
58
60
68
72
74
76
82
84
86
88
94
C2H4O2
C2H4O2
C2H4O2
c4h4o
c4h8o
C3H402
ch2=chcooh
c3h602
HOCH2COMe
c3ha
H02CEt
C3H602
HOCH2CH2CHO
c3h6o2
c2h4o3
ch3cooch3
c5h6o
c6h8o
V
MeCOCOMe
MeCH=CHCOOH
VI
H02CEtMe
c6h6o
PhOH
C5Hg02
c5h8o2
MeCOCOEt
C5H10O2
CH3(CH2)3COOH
2-methyl-2-cyclopentene-l-one
furfuryl alcohol
5-methyl-2(3H)-furanone (a-angelicalactone)
3-methyl-2(3ff)-furanone
valerolactone
2,3-pentanedione
pentanoic acid (valeric/isovaleric)
c7h8o
ch3c6h4oh
o-cresol
c5h4o2
C5H602
c6h6o
100
102
108
110
112
XV
XVI
XVII
CH3(CH2)4C02H
CH3C02CH2(C0)CH3
hexanoic acid
l-acetyloxy-2-propanone
c8h8o
XVIII
C8H10O
C6H3(CH3)2OH
C6H4(CH2CH3)OH
vinylphenol
dimethylphenols
ethylphenol
c6h6o2
c6h6o2
c6h8o2
c6h6o3
C6Hi202
c5h8o3
120
122
C8Hi0O
124
126
132
138
146
150
152
c7h6o2
c7h8o2
C8Hi20
CgH803
c6h8o3
c6h8o4
C8Hi0O2
CgHi0O4
C9H10O2
C9H10O2
C9Hio02
c8h8o3
154
162
164
166
168
178
180
furfural
m/p-cresol
5-methylfurfural
dihydroxybenzenes (catechol, hydroquinone, resorcinol)
2-hydroxy-3-methyl-2-cyclopentene-l-one
dimethylcyclopentanone
3-hydroxy-2-penteno-l, 5-lactone
C7H120
114
116
MeCh2COCH2OH
VII
VIII
IX
X
XI
XII
XIII
c6h8o
98
II
III
IV
c4h4o2
c4h6o2
c4h6o2
c4h6o2
c4Hg02
c4h6o2
C7H10
96
HOCH2COOH
c8h10o3
C8Hi<A
Ci0H12O2
CgHioOs
CgHi203
Ci0Hi0O3
Ci0H12O3
CioH1203
C6Hi206
XIV
C6H4(OH)2
c6h6co2h
XIX
XLIV
XX
XXI
MeCOOCH2(CO)CH2OH
XXII
MeCOO(CH2)2(CO)CH2OH
XXIV
XXV
XXVI
XXVII
XXVIII
XXIX
XXX
XXXI
XXXII
XXXIII
XXXIV
XXXV
XLV
benzoic acid
2-methoxyphenol (guaiacol)
trimethylcyclopentenone
5-(hydroxymethyl)-2-furfural
2-methyl-3-hydroxy-4-pyrone
l-hydroxy-2-propanone acetate
4-methylguaiacol
l-hydroxy-2-butanone acetate
p-vinylguaiacol
coumaryl alcohol
4-ethylguaiacol
vanillin
major
ion*
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
f
f
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
b, e
b-e
e
a-f
a-f
a
g
a, c-e
e
a-f
c-e
a, b
e
d
a, b
e
X
e
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
a-d
b
a, b, e
e
e
c-e
c, e
a
c, e,
f
d} e
d, e
a, e
a
d, e
e
c-e
a, d
a
f
c, d,
c, d
b, c, e
a
g
c, d
a
g
X
X
X
X
X
X
X
X
X
c, d
d
a
a-e
a
g
d
e
a, b, e
e
g
g
X
X
X
a, b, d, e
f
X
X
X
X
X
X
X
X
2,6-dimethoxyphenol (syringol)
a-f
X
X
levoglucosan
isoeugenenol
eugenol
a-D-glucose
/
X
X
X
X
X
X
chem
anal, of
oils
X
X
X
X
l-(4-hydroxy-3-methoxyphenyl)ethanone
4-methyl-2,6-dimethoxyphenol
coniferyl aldehyde
coniferyl alcohol
vinylsyringol
in
product
at >0.1%*
X
a
X
X
g
a, b, d, e
c, d, f
X
g
d
a
X
X
a, e
a
X
g
X
X
g
X
d
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987
129
Table I (Continued)
ref to
ion,
m/z
182
formula
structure
likely product
C9H10O4
XXXVI
XXXVII
XXXVIII
XXXIX
XL
XLI
XLII
XLIII
syringaldehyde
4-ethylsyringol
4-propenylsyringol
ferulic acid
(4-hydroxy-3,5-dimethoxyphenyl)ethanone
sinapyl aldehyde
sinapyl alcohol
CioH1403
194
196
208
210
272
major
ion*
ChH1403
CioHi004
C10H12O4
CuH1204
ChH1404
C16H16O4
in
product
at >0.1%‘
X
X
X
X
X
chem
anal, of
oils
a-d
a
a
g
X
a
X
g
X
g
X
g
“Reference 38. GC/MS and GC of a vacuum pyrolysis oil from the University of Sherbrooke (poplar). bReference 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
d
pyrolysis oil (poplar). Reference 40. A flash pyrolysis oil from the University of Waterloo (poplar). 'Reference 39. GC/MS of oils from a 1-atm
pyrolysis of powdered beechwood. /Reference 45. GC analyses of hybrid poplar, pyrolyzed at 504 “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 /.
Scheme I
CELLULOSE CHAIN
R =
/
OH
0
I
II
HOCHjCH
R-O'
C=C
CARBONYL
CROSS-LINKING
been discussed in past work,10 but in general these effects
not large32 in 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 stage42 or
“active cellulose”43 stage, which corresponds to a decrease
in DP from 1000-2000 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
°C.8 At this temperature cellulose undergoes transglycosylation to form levoglucosan in high yields.3,42 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. Adv. Carbohydr. Chem. Biochem. 1977,
34, 23-177.
COMPOUNDS
BETWEEN CHAINS
are
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%.8 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
wt % potassium or sodium as the carbonate or hydroxide
inhibits the formation of levoglucosan and leads to a different product state composed of furfural derivatives (m/z
126,110, 96), low molecular weight carbonyl compounds
represented by m/z 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 1 to 10%. Our
work over the past several years has led to the speculative
mechanism shown in Scheme II for 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
more
130
Evans and Milne
Energy & Fuels, Vol. 1, No. 2, 1987
Chart I
0
II
1
0
n
II
£
£
0r°
cr
HCsf/XfC1
m
IV
V
VI
VII
Xltl
XIV
0
00^CH,
\I
LJ
VIII
IX
fir*'
H3CY°^Q
CXxh3
XI
XII
X
o
o
XXI
'Me
Mel
HO
XXVII
XXVIII
XXXIV
XXXV
HO
XXXII
XXXIII
o
II
CH=CH-CH
CH=CH-CHpH
iMe
MeO'
HO
XLI
al.46 proposed that the monomer
unit of cellulose decom-
poses 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
at 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 at 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.
XLII
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 0-2 position.
In separating the xylan from wood, the acetyl group is
cleaved and as a result the pyrolysis spectrum for xylan
shows little acetic acid (m/z 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 amu 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 m/z 132 would be 1,4-anhydroxylanopyranose. Previous work33 has shown that the m/z 144 abundance for
pure cellulose pyrolysis is largely due to ionization fragmentation and the species at m/z 114 may also be in part
due to the ionization fragmentation of the m/z 132 species.
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987
131
However, Ohnishi et al.51 in a study of xylan pyrolysis,
found a product at m/z 114 and identified it as 3hydroxy-2-penteno-l,5-lactone (XVII). This compound
was also identified by Van der Kaaden et al.62 from the
pyrolysis of amylose. Schulten et al.,53 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 at 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., m/z 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 at 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 at 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 at m/z 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 (m/z 164), vanillin (m/z 152), and vinylguaiacol (m/z 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 (m/z 124), and catechol
(m/z 110). These products tend to reach their maximum
evolution rate at the end of pyrolysis as shown by m/z 138
in Figure 2. This is consistent with the observation in
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 m/z 150 in the bagasse sample in Figure 3.
Vinylphenol (m/z 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 Bource 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 II and III show prominent
secondary cracking peaks that grow at the expense of
primaries. Many of these have been seen in liquids subjected to high cracking severity (see below and55). One
can distinguish a “primary oxygenates” zone from 400 °C
(same spectrum as 500 °C) to about 700 °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
(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.
ical Conversion 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.
(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 1985 Biomass Thermochem-
132
Evans and Milne
Energy & Fuels, Vol. 1, No. 2, 1987
100 r
100 r
Cellulose 950° c
Sweet gum lignin 950° c
178
25
59
lSi
75
2tt
175
25«
225
'
275
3M
25
58
?5
J^U.
188
125
282
^
i
27b
i4 "“ii
100-r
Cellulose 700° c
<A
Sweet gum lignin 700° c
(A
C
C
CD
CD
0)
_>
0)
>
o
cc
ok
u
0)
QC
^188^+1"23
'1
^iSV
175
r
28e
r
T
'
’^25
2&8
'
38e
V75
100
Sweet gum lignin
500° c
Cellulose 500° c
98
jLJlijU-
1
15S
i75
2M
225
^'
250
^75
1
3M
m/z
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 °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 II. Prominent Intermediate Ion Peaks in the
Secondary Cracking Regime of Wood
formula
ion, m/z
possible compd
16
methane
ch4
28
30
40
42
44
54
56
66
92
94
104
106
108
110
118
120
122
124
130
132
136
142
144
148
160
168
182
c2h4
c2h6
c3h4
c3h6
co2
C4H6
c4h8
c5h6
cvh8
c6h6o
c8h8
CsHio
c7h8o
c6h6o2
c8h6o
c8h80
c7Hg02
c7h802
c9h8o
c8h8o2
CgH802
ethene
ethane
propyne
propene
carbon dioxide
butadienes, butyne
butenes
cyclopentadiene
toluene
phenol
styrene
xylene
cresol
dihydroxybenzene
benzofuran(?)
vinylphenol
benzodioxole (?)
dihydroxytoluene, guaiacol
methylbenzofuran (?)
methylbenzodioxole (?)
vinylbenzodioxole (?)
The spectra at 500 °C for the four materials shown in
Figure 4 and 5 are dominated by peaks that are all due to
primary products; however, there is some decrease in the
Table III. Prominent Growth Ion Peaks in the Tertiary
Cracking Regime of Wood
ion, m/z formula
possible compd
16
methane
ch4
18
water
h2o
26
28
44
52
66
78
92
94
104
116
128
152
166
178
202
216
226
228
c2h2
CO
co2
C4H4
c6h6
c6h6
c7h8
c6h6o
c8h8
c9h8
CioHs
Ci2H8
Ci3H10
C14H10
CieHio
Ci7H12
^18^10
c18H12
240
252
c19h12
276
(J22hli2
^2olii2
acetylene
carbon monoxide
carbon dioxide
vinylacetylene (?)
cyclopentadiene
benzene
toluene
phenol
styrene
indene
naphthalene
acenaphthylene
fluorene, benzindenes
anthracene, phenanthrene
pyrene, fluoranthene, benzacenaphthylene
methylpyrene
benzo[ghi] fluoranthene
chrysene, benz[a]anthracene, triphenylene,
benzo [ c ] phenanthrene
methylbenzo[ghi] fluoranthene
benzofluoranthenes, benzo[ae]pyrene,
perylene
anthanthrene et al.
relative abundance of the heavier species. No major
change in composition occurs until approximately 550 °C
where the first species to crack are the alkenyl aromatics,
such as coniferyl alcohol. A significant decrease in m/z
180 and 210 is already observed at 500 °C for sweet gum
MWL in Figure 4.
Pyrolysis of Biomass
Energy & Fuels, Vol. 1, No. 2, 1987
133
100t
Xylan 950°
152
178
202
*
'
25
s&r'
75
iM'
'
125
ise1
1_2(w11
ias^asV'^afs
3W
<0
c
03
03
_>
03
03
cc
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 at 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 Lai57 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, shown 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
C02 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
products are essentially 30 amu less than those from cel-
lulose (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 (1-propenyl-), and 178 (l-propen-3-al).
Past work54 on the cracking of vanillin (m/z 152) has
shown a major peak at m/z 150, and the cracking of isoeugenol (m/z 164) gave a peak at m/z 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-
134
Evans and Milne
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 spectrum) since
plots of intensity vs temperature show these species increasing from low values at 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 at 700 °C.
The tertiary products are shown at 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 C02, but the formation of
benzene and the heavier aromatics such as naphthalene
(m/z 128) and anthracene (m/z 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 Antal9 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,56 and 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 al.41,58,59 in 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 first factor is the linear combination
of masses that accounts 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”,
ay, 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:
Fj
=
+ anjZnj
aljZ1 + a2jZ2 +
j 1 -m; m < n
...
-
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 Meuzelaar41 developed 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
al_41,58,59
The secondary cracking of pine wood pyrolysis products
studied at 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 al.60 Ten factors were extracted from the data, which together accounted for 98.2%
of the original variance. The plot of factor score 1 vs 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 1 and 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 1 and 2 (340° rotation) shows
that this represents the primary products, and it is logical
that the low-temperature samples will be located in this
was
(60) Windig, W.; Chakravarty, T.; Richards, J. M.; Nguyen, V. T.;
Dedes, A.; Meuzelaar, H. L. C. “SIGMA, an IBM 9000 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
Energy & Fuels, Vol. 1, No. 2, 1987
135
43
b)
Secondary
90°
F2
270°
Figure 6. Factor analysis of mass spectrometric data for the effect
of gas phase temperature and residence time on product distribution: (a) plot of factor score 1 vs. 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 Fx and F2 that best describe the chemical components. The factor spectra for these components are shown in
Figure 7.
region. This component axis at 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 at 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 at 120°). 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, m/z 30;
propylene, m/z 42; butadiene, m/z 54), the phenols (m/z
94, phenol; m/z 108, resol; m/z 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 at 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
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 C02, 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,
136
Evans and Milne
Energy & Fuels, Vol. 1, No. 2,1987
Primary processes
Secondary processes
Tertiary processes
Vapor
phase**
Liquid
phase**
Solid
phase
Pyrolysis severity‘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
Irvine61 has shown that wood has a glass transition temperature, at 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”
The primary, true liquids
wood is postulated to occur.
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
al.64
The upper, vapor-phase, pathway in Figure 8 is the
(61) Irvine, M. G. TAPPIJ. 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
above,61 lignin is known to soften at 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 workers23 are primarily C02, H20, and
CO. These are largely associated with the char forming
reactions.5 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
vacuum.20,53 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 °C 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 at
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 III. 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 series,62 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 Antal 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; II, 534-22-5; III, 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, 6638-05-7; 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;
XLH, 537-33-7; XLIV, 106544-45-0; 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, 64-18-6; MeCOMe, 67-64-1; H02CMe,
64-19-7; HOCH2CHO, 141-46-8; MeOCHO, 107-31-3; HOCH2CH2CHO, 2134-29-4; EtCOMe, 78-93-3; CH2=CHCOOH, 79-10-7;
HOCH2COMe, 116-09-6; H02CEt, 79-09-4; CH3COOCH3, 79-20-9;
HOCH2COOH, 79-14-1; MeCOCOMe, 431-03-8; MeCH=
CHCOOH, 3724-65-0; H02CEtMe, 107-92-6; MeCH2COCH2OH,
5077-67-8; PhOH, 108-95-2; MeCOCOEt, 600-14-6; CH3(CH2)3C-
OOH, 109-52-4; o-CH3C6H4OH, 95-48-7; m-CH3C6H4OH, 108-39-4;
p-CH3C6H4OH, 106-44-5; C6H4(OH)2,12385-08-9; CH3(CH2)4C02H, 142-62-1; CH3C02CH2(C0)CH3, 592-20-1; C6H3(CH3)2OH,
1300-71-6; C6H4(CH2CH[3)OH, 25429-37-2; C6H5C02H, 65-85-0;
CjHg, 74-84-0; C3H4, 74-99-7; C3H6,115-07-1; C2H2, 74-86-2; C6He,
71-43-2; K2C03, 584-08-7; K, 7440-09-7; l-hydroxy-2-propanone
acetate, 106544-46-1; l-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, 208-96-8; 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; benzo[c]phenanthrene,
195-19-7; methylbenzo[ghi]fluoranthene, 51001-44-6; benzofae]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.