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Fuel 83 (2004) 1469–1482
Characterization of chars from pyrolysis of lignin
Ramesh K. Sharma, Jan B. Wooten, Vicki L. Baliga, Xuehao Lin1,
W. Geoffrey Chan*, Mohammad R. Hajaligol
Philip Morris USA Research Center, P.O. Box 26583, Richmond, VA 23261-6583, USA
Received 6 June 2003; revised 25 November 2003; accepted 25 November 2003; available online 4 March 2004
The characterization of lignin char and its reactivity towards the formation of polycyclic aromatic hydrocarbons (PAHs) were studied. The
char was prepared by pyrolyzing lignin at atmospheric pressure and temperatures ranging from 150 to 550 8C under both pyrolytic and
oxidative (5% oxygen in helium) atmospheres. The chemical composition of char was characterized by Fourier transform infrared
spectroscopy (FTIR), cross-polarization magic angle spinning 13C nuclear magnetic resonance spectroscopy (CPMAS 13C NMR), and
energy-dispersive X-ray spectroscopy. The surface area of the char was measured by Brunauer – Emmett– Teller method and surface
morphology obtained by scanning electron microscopy (SEM). The char yield in pyrolysis decreased rapidly with an increase in temperature
until 400 8C, after which there was a gradual decrease in the yield to ca. 40% at 750 8C. In oxidative atmosphere, the char yield decreased to
ca. 15% at 550 8C. The surface area of the char was low with a maximum of 5 m2/g. SEM analysis indicated that the pyrolysis led to the
formation of melt, liquid phase, vesicles, precipitates of inorganic salts and surface etching. These structures decomposed rapidly at high
temperatures. FTIR and NMR studies showed a gradual decrease in the amounts of OH and CH3 with increasing temperature. At 550 8C,
most IR bands, except those due to the aromatic CH and OH stretches, disappeared resulting in mainly aromatic char. Both the H/C and O/C
ratios of the char decreased with increase in temperature. Although the data on char reactivity was limited, it indicated that surface area,
presence of inorganics and aromaticity of char may be important factors in PAH formation. These chars have low reactivity, compared to
chars from other biomass constituents, such as chlorogenic acid, pectin and cellulose, probably due to the highly cross-linked and refractory
nature of the lignin char.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Combustion; Pyrolysis; Char; Characterization; Lignin; Polycyclic aromatic hydrocarbons
1. Introduction carbon-to-carbon bonds. The content of lignin ranges from
20 – 40% in wood and ca. 3% in the tobacco leaf.
Lignin is the most abundant polymeric aromatic organic One of the methods of extracting lignin is the treatment
substance in the plant world . Lignin occurs together with of wood with an alkali such as sodium hydroxide or a
cellulose and other polysaccharides in the cell walls of the mixture of sodium hydroxide and sodium sulﬁde at elevated
temperatures (170 – 180 8C). The dissolved ‘alkali’ lignin is
plants, particularly in woody plants where lignin accumu-
then precipitated with sulfuric acid. The alkali lignin is
lates between the cellulose microﬁbrils in the middle
different from ‘milled-wood lignin’, which is obtained by
lamella and the primary and secondary walls of the xylem
extracting wood with a dioxane – water mixture.
elements. Unlike cellulose, lignin is a highly cross-linked
A number of studies have been reported in the literature on
polyphenolic polymer without any ordered repeating units.
pyrolysis of lignin [3 –15]. The pyrolysis generally leads to
A typical structure of lignin  is shown in Fig. 1. The most
the formation of a volatile product and a solid residue, i.e.
common monomeric unit in lignin is a phenylpropanoid unit char. The relative distribution of products is dependent on
linked to other units via mainly ether linkages as well as pyrolysis conditions. Schlotzauer et al.  reported the major
components of the volatile product from pyrolysis of lignin at
* Corresponding author. Tel.: þ 1-804-274-5865; fax: þ1-804-274-2160. 700 8C were substituted methoxyphenols. The highest rate of
E-mail address: email@example.com (W.G. Chan). formation of phenols occurred in the 500 –600 8C region. The
Present address: 2835 S. Quinn St, Chicago, IL 60608, USA. yields of guaiacol, phenol, and m- and p-cresol were highest
0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
1470 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
Fig. 1. A proposed structure of beech lignin.
at 500 8C. The amount of phenol was independent of the demethoxylation and recombination of the primary radicals
source of lignin but the amount of m- and p-cresol was much whereas ZnCl2 promoted the ionic decomposition route.
lower from tobacco lignin than that from the wood lignin. As literature indicates that considerable work has been
Similar results were obtained under aerobic conditions done on the composition of the volatile products, there is no
except that the yields were lower. detailed information on the nature of char formed in the
Caballero et al.  studied pyrolysis of Kraft lignin in a pyrolysis of lignin. The char is the intermediate solid
Pyroprobew at temperatures from 450 to 900 8C with residue, which is formed in the pyrolysis of most biomass.
heating rates of 20 8C/ms. The major gaseous products were The char is believed to contribute to the formation of
methanol, formaldehyde, acetaldehyde, acetic acid and light polycyclic aromatic hydrocarbons (PAHs) during biomass
hydrocarbons, in addition to CO, CO2, and H2O. Among the pyrolysis, particularly at low temperature . The chars
hydrocarbons, the highest yield was for methane, followed from pectin and tobacco [16,34] were studied and it was
by ethylene and benzene. The yields increased sharply found that the physical and chemical characteristics of chars
above 500 8C. Alen et al.  also used a Pyroprobew to were governed more by pyrolysis temperature than the
pyrolyze softwood lignin and observed a decrease in char nature of the substrate. Since lignin is an important
yield from 70% at 400 8C to ca. 20% char yield at 1000 8C. constituent of woody biomass it is of interest to determine
The volatile product contained mainly vanillins and how the composition and the aromatic structure of lignin
guaiacols at 400 8C and aromatic hydrocarbons and phenols affect the characterization and reactivity of char from lignin.
at 1000 8C. Catechol was observed at 600 –800 8C. In this study, the char was prepared by pyrolyzing lignin
Nunn et al.  used a screen heater to pyrolyze milled at atmospheric pressure and temperatures ranging from 150
wood lignin at 300 – 1100 8C and observed methanol, to 550 8C under both pyrolytic and oxidative (5% oxygen in
formaldehyde, acetaldehyde and acetic acid in the product, helium) atmospheres. In some runs, temperatures of up to
similar to that reported by Caballero et al. . Iatridis and 750 8C were used. The chemical composition of the char
Gavalas  reported similar results from the pyrolysis of was characterized by Fourier transform infrared spec-
Kraft lignin in a screen heater at 400 –700 8C. troscopy (FTIR), cross-polarization magic angle spinning
Britt et al.  reported that lignin pyrolysis occurred C nuclear magnetic resonance spectroscopy (CPMAS 13C
mainly by a free-radical reaction mechanism. Jakab et al.  NMR), and energy-dispersive X-ray spectroscopy (EDS).
used a thermogravimetric analyzer (TGA) to pyrolyze The surface area of the char was measured by Brunauer–
various lignins in the presence of inorganics such as NaCl Emmett – Teller method (BET) and surface morphology
and ZnCl2. NaCl was found to promote dehydration, obtained by scanning electron microscopy (SEM).
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1471
The reactivity of char towards the formation of PAHs was spectrometer. The pyrolysis and evolution of most com-
measured by pyrolyzing the char at 700 8C. The results were ponents were generally complete in 10 min. This run
compared to those from other biomass constituents, such as duration also ensured that no signiﬁcant decomposition
chlorogenic acid, pectin, cellulose and tobacco. occurred during the cooling of the sample. At the end of the
run, the product char was allowed to cool to ambient
temperature before being recovered. It was stored over dry
2. Experimental silica gel under vacuum until analyzed. The char yield was
calculated from the amount of char based on the initial mass
The lignin used in this work was ‘alkali’ lignin, of the unpyrolyzed lignin. The experimental error in the
obtained as a powder from Aldrich (Milwaukee, WI). It is yield measurements was less than ^ 1%.
a polymeric Kraft lignin with a molecular weight of ca.
142,000. The particles were dark brown in color and had a 2.2. Char characterization
glassy, translucent quality. The analysis of the as-received
lignin showed that it contained 4.6% water and 5.7% ash. The elemental analysis of char was performed at Galbraith
On a dry, ash-free (daf) basis, the carbon, hydrogen and Laboratories, Inc. (Knoxville, TN). The surface area was
sulfur contents were 66.5, 5.8 and 1.9%, respectively. Since measured in an automated volumetric gas adsorption
the content of nitrogen was , 0.5%, the oxygen content is apparatus (Autosorb 1 from Quantachrome Co.) using
assumed to be 25.3%. The ash consisted mainly of oxides of nitrogen as an adsorbate at 77 K. Prior to adsorption
sodium (37.2%), potassium (7.5%), silicon (3.3%) and measurements, the sample was outgassed at 120 8C for 2 h.
aluminum (1.9%), along with small concentrations Typically, 400 mg of char was used in each area
(, 0.4%) of magnesium, calcium, iron, phosphorus and measurement.
titanium. The surface morphology of the char was studied by SEM.
For SEM analysis, the char sample was analyzed mostly
2.1. Pyrolysis procedure while still in the porcelain boat to maintain the integrity of
the surface. A coating of 8 nm Au/Pd ﬁlm was applied to the
Pyrolysis was carried out in a tubular reactor, and the sample using a Cressington 208 HR sputter coater. The
experimental details have been given elsewhere . coated samples were then examined and imaged using JEOL
Brieﬂy, the reactor consisted of 0.5 in. diameter quartz 840 SEM. Elemental determinations of the char surfaces
tube heated by a sliding 6 in. long stainless steel block were obtained with either the Princeton Gamma Tech EDS
furnace. The furnace provided about a 4-in. length of or Kevex EDS.
uniform (^ 5 8C) temperature proﬁle. A chromel/alumel Infrared spectrum of char was recorded on a Spectra-
thermocouple was placed inside the tube, embedded in the Tech IR-Plan microscope interfaced to a Nicolet Magna 560
sample, to measure the temperature. The carrier gas was FTIR spectrometer. The sample was held between two KBr
helium in pyrolysis runs and a mixture of 5% oxygen in salt plates in a microcompression cell. In some cases,
helium in oxidative runs. The concentration of oxygen in the preﬂattening in a diamond cell was necessary prior to
oxidative runs was kept low to prevent the complete mounting. A spectral resolution of 4 cm21 was used, and the
oxidation of lignin. apodization function was of the Happ-Genzel type. Due to
A porcelain boat containing up to 500 mg of lignin, differences in the IR signal from various samples and to
spread as a thin layer, was placed about 3 in. from the optimize the IR signal from each sample the amount of
downstream end of the tube. The pyrolyzing gas was sample used in IR could not be kept constant. The spectra
introduced at a ﬂow rate of 220 N cm3/min that corre- were analyzed to compare the relative intensities of a
sponded to a residence time of about 500 ms at 250 8C and functional group in different chars. However, since the
350 ms at 550 8C. The furnace initially rested over an empty amount of sample used in different analyses was not the
portion of the tube and was equilibrated at the desired same, only the relative ratios of the various functional
temperature. After the temperature was reached, the furnace groups in different chars were compared.
was moved over the sample to initiate pyrolysis. The runs Solid-state 13C CPMAS NMR spectrum of char was
were made at atmospheric pressure and at temperatures obtained on a Varian Unity 200 spectrometer at a carbon
ranging from 150 to 750 8C under approximately isothermal resonance frequency of 50.3 MHz. The NMR probe was a
conditions (except for the initial heating time). The sample Doty Scientiﬁc (Columbia, SC) high-speed magic-angle
reached within 5 8C of the desired temperature in ca. 4 min. spinning probe. The MAS spinning speed was , 8100 Hz,
The product gases were passed through a Cambridge pad fast enough to move the spinning side bands out from the
before being vented into a hood. A sample of product gases region of interest. The cross-polarization contact time was
was analyzed on-line by a Balzer QMG 511 quadrupole 750 ms. The Hartmann – Hahn spin-lock match was made on
mass spectrometer. The spectrometer was operated at 24 eV the aromatic peak for all samples. The pulse repetition rate
energy to minimize molecular fragmentation. The run was 2 s. The curve analysis program is a component of
duration was based on the total ion current in the mass Vnmr 6.1c.
1472 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
2.3. Char reactivity the Pyroprobew. The ultimate char yield in this study is also
much higher than that from pectin or tobacco , which was
Three different chars, each prepared at 300 8C (Char #1), ca. 20% at 750 8C.
350 8C (Char #2), and 400 8C (Char #3) were used in the Fengel and Wegener  suggested that the naturally
reactivity study. The char was pyrolyzed at 700 8C in occurring inorganic species in wood catalyze the pyrolysis
helium. The product was passed through a Cambridge pad to of wood. In order to determine if inorganics play any role
collect the condensable tar, which was analyzed for PAHs during the pyrolysis of lignin in this study, a sample of
by an Agilent 6890/5973 gas chromatograph/mass speciﬁc lignin was washed/extracted with acid to partially remove
detector (GC/MSD). the inorganics. The extraction led to a decrease in the
amount of ash to 1% of lignin and the concentrations of
sodium and potassium decreased to 1.3 and 0.7% of ash,
3. Results and discussion respectively. When the extracted sample was pyrolyzed, the
char yield decreased from ca. 71% of lignin (on a daf basis)
3.1. Char yield for the unextracted sample to 51% for the extracted sample
at 300 8C and to 36% above 600 8C. This indicated that the
Fig. 2 shows the effect of temperature on char yield. The partial removal of sodium and potassium enhanced the
term ‘char’ is used to represent the solid residue that de-volatilization of lignin at the expense of char formation.
remained after pyrolysis. It consisted of organic material DeGroot and Shaﬁzadeh  observed a similar decrease in
with a composition varying from barely pyrolyzed lignin at the char yield from wood after the wood was acid-washed. It
low temperatures to a highly carbonized material at high has been reported in the literature that sodium promotes the
temperatures. The char also included any coke that might demethoxylation, demethylation and dehydration of lignin
have been formed by reactions among the volatile com- [9 –11]. Thus, the high char yields in this study may be due
ponents. The char yield decreased rapidly with increasing to the presence of inorganic components in lignin.
temperature to ca. 62% (on daf basis) at 400 8C, followed by a An interesting aspect of the results in Fig. 2 is the wide
gradual decrease to 40% at 750 8C. These yields are 5– 10% temperature range in which the lignin pyrolysis occurred.
higher than those reported by Caballero et al.  from The decomposition began at temperatures below those
pyrolysis in a Pyroprobew at 450– 900 8C. It should be noted reported for carbohydrates such as cellulose and hemi-
that the actual temperatures in a Pyroprobew could be cellulose  and pectin  but a signiﬁcant fraction of
different than the set points since heat transfer occurs from lignin remained as char even at temperatures above those at
the platinum coil through the quartz tube to the sample. Char which the carbohydrates were completely pyrolyzed. The
yields are known to increase with a decrease in the heating char yield in this study was independent of the sample size,
rate . Thus, the higher char yields in this study may be due indicating that any condensation reactions among the
to the relatively lower heating rates compared to those in volatile products, leading to the formation of coke, were
absent under the above pyrolysis conditions. Such reactions
generally increase the char yield .
The oxidative atmosphere led to char yields that were
identical to those in the pyrolysis runs until 350 8C but
considerably lower at higher temperatures. This shows that
the extent of oxidation of lignin was small below 350 8C,
probably due to a low reactivity of the solid matrix. Since
the lignin forms a melt above 250 8C, as indicated by the
SEM results (presented later), the bulk of pyrolysis probably
occurred along the thin layer at the condensed phase/gas
phase interface. The bubbles that grew within the melt
transport the volatiles towards the surface. In oxidizing
atmosphere, oxygen diffusion through the melt layer might
have been favored by the large holes formed at the surface
by the bursting of bubbles due to the release of the volatile
products. Similar observations were made by Kashiwagi
and Ohlemiler . At 550 8C, ca. 20% of lignin remained
as char in this study indicating a rapid oxidation of lignin or
its char in the presence of oxygen.
Preliminary analysis showed that the apparent acti-
vation energy for char formation in pyrolysis was low,
Fig. 2. Effect of temperature on char yield from lignin under pyrolytic and 76 kJ/mol. This value is in the range of the activation
oxidative conditions. energies, 37 – 96 kJ/mol, reported in the literature for
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1473
the pyrolysis of various lignins using different kinetic of the gaseous product is also affected by the residence
models [4,6,14,21,22]. Antal and Varhegyi  pyrolyzed time, in addition to temperature.
cellulose and observed that with increase in the rate of
heating of cellulose, the activation energy decreased 3.2. Char characterization
probably due to increased transport limitations. It should
be added that the pyrolysis mechanisms are complex since 3.2.1. Surface area
lignin has a complex structure and contains a large number
The BET surface areas of the various chars from the
of organic and inorganic groups. The pyrolysis may
pyrolysis of lignin are presented in Fig. 3. The surface area
encompass two extreme regions: a chemical-controlled
was generally low with a maximum of , 5 m2/g at 350–
region at low temperatures and a transport-limited region at
400 8C. The high temperatures were detrimental to the pore
high temperatures. The apparent activation energy in this
structure of the char. The surface area values are
study probably represents the energy for the rate-controlling
comparable to that from tobacco but lower than those
step and includes the effect of any transport limitations.
from chlorogenic acid or pectin [34,35]. The maximum
Although the main focus of this work was on the char
surface area from lignin is obtained at a temperature slightly
characterization, the major volatile components were also
lower than for the other substrates. The above results
identiﬁed in some runs. The components included H2O, CO,
suggest that either the lignin chars are not microporous or
and CO2 as well as those with m=z of 58, 60, 74, 76, 94, 124
that the pores within these chars are extremely small and
and 251 at low temperatures. The largest yield was that of
dead-ended, preventing any access to the adsorbing gas. The
m=z ¼ 58; followed in decreasing order by m=z of 94, 124,
microporous samples with open pores are expected to have
60, 76, 74 and 251. Based on the literature results discussed
high surface areas. The low areas could also be due to the
earlier and our own previous work [16,34], the components
formation of a plastic-like char as a result of the melting of
represented mainly phenols and aromatic acids, such as
lignin indicated by SEM results. As the pyrolysis tempera-
phenanthrenecarboxylic acid, hydroxymethoxy benzenea-
ture increased, the evolution of volatile products increased.
cetic acid, dihydroabietic acid, dimethoxyphenyl ethanone,
This will lead to an increase in the formation of bubbles and
vanillin, catechol, methoxy phenol, methyl catechol and
pores in the melt and the surface area of the char.
dimethoxy phenol. Although no attempt was made to
determine the individual yields, relatively, the yields of the
most components decreased with increase in temperature 3.2.2. SEM analysis
above 350 8C, except the yields of H2O, CO and CO2, which The results of SEM analyses of lignin and its chars are
increased. Other components with m=z of 64, 79, 96, 131 and shown in Figs. 4 and 5. Only the chars formed by pyrolysis
185 were also observed. In addition, the product at 550 8C were analyzed. The analyses showed that the individual
contained components with m=z of 2, 78 and 92 (which may particles of lignin were polygonal in shape with multiple
represent hydrogen, benzene and toluene, respectively), conchoidal fracture surfaces (Fig. 4a, arrow).
along with components with m=z of 62, 64 and 126 in small Analysis of the char at 250 8C showed that the lignin
yields. The largest yield at 550 8C was for hydrogen. particles had softened, melted and fused into a mass of
Interestingly, hydrogen and benzene were observed even in matrix and vesicles (Fig. 4b). The vesicles were the result of
oxidative runs at 550 8C. These results indicate that the volatile gasses released within the softened lignin matrix. At
major pyrolysis step was dehydration, with some decarbox- 250 8C, the vesicles maintained the expanded shape after
ylation, and decarbonylation, with the aromatic rings cooling (Fig. 4b).
remaining essentially intact. Some dehydrogenation of
lignin also occurred at high temperatures. Considering the
high char yields from lignin, it appears that the volatile
products originated mainly from side groups of the lignin
polymer, without affecting the structure of the polymer.
Evans et al.  pyrolyzed ball milled lignin at 550,
650 and 750 8C with residence times of 50 –225 ms. The
major product components at 750 8C were those with m=z
of 60, 66, 78, 94, 110, 124 and 136, compared to m=z of
124, 138, 168, 180, 194 and 210 at 550 8C. The exact
identity of the compound corresponding to these masses
was not reported. In the presence of 5% oxygen, the
concentrations of m=z of 94, 110, 124 and 136 decreased
signiﬁcantly at 750 8C but the effect of oxygen on
concentration was small at low temperatures. The
residence time of volatiles in the reactor in this study
was relatively higher, 350– 500 ms. Thus, the composition Fig. 3. Effect of temperature on surface area of char.
1474 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
Fig. 4. SEM micrographs of lignin (a) and its pyrolytic chars prepared at 250 8C (b), 300 8C (c) and 350 8C (d, e).
As temperatures increased, more vesicle formation was temperatures, the remains of the crystals were identiﬁed
observed in the lignin. Some vesicles at around 300 – 350 8C only by sodium- and/or sulfur-rich areas. Regions rich in
exhibited very thin and pliable membrane coverings, which potassium became prominent at around 550 8C and higher.
deﬂated after cooling (Fig. 4c – e). The char morphology This suggested that inorganic elements were mobile within
exhibited both open and closed vesicles throughout the the lignin melt followed by precipitation. Oxygen was
temperature range (Fig. 5a –c). For chars prepared above detected in some of the crystals, which suggested that the
550 8C, portions of the matrix appeared brittle with planar oxygen-containing crystals might be oxides or carbonates.
(Fig. 5d, arrow) and conchoidal fractures. Other regions At temperatures from 550 8C and higher, acicular or needle-
showed signs of plastic deformation with new vesicle shaped crystals started to appear (Fig. 5c –e, arrows). They
formation at temperatures up to 750 8C (Fig. 5e, arrows), the seemed to develop from equant-shaped particles on the
last temperature that samples were collected. This might be lignin matrix. Both the equant-shaped particles and the
expected according to the ﬁnding of Orfao et al.  who acicular particles may be the result of secondary product
showed that lignin continued to degrade up through formation from the precipitation of volatile gases. Inorganic
1000 8C. Recently, Baliga et al.  compared the physical and carbonaceous precipitates were present on much of the
morphology of char from tobacco and its components, exposed surfaces at 650 and 750 8C except in regions of new
including lignin. It was found that lignin-rich xylem vesicle formation (Fig. 5e, arrows). The physical mor-
elements from tobacco did not melt until after 300 8C. phology is discussed in greater detail elsewhere .
This showed that interactions among components of the
mixture, such as the cell wall matrix with lignin and 3.2.3. FTIR analysis
inorganic salts, affected the pyrolysis behavior of the The lignin and its chars were characterized by infrared
various constituents. spectroscopy in the near IR region (wave numbers:
The alkali lignin used in this experiment contained 4000– 600 cm21). A typical IR spectrum of lignin is shown
inorganic crystals throughout the matrix. Euhedral, pyr- in Fig. 6. Various bands in the spectrum were identiﬁed as
amidal-shaped crystals that contained sodium and sulfur corresponding to OH (at wavenumber of 3419 cm21),
were present in the starting lignin up through 350 8C methoxyl (2844 cm21), aliphatic CH (3000 – 2860 cm21),
(Fig. 4e). At 450 8C, the crystals began to show signs of and aromatic CH (3064 cm21) groups as well as aromatic
melting and degradation (Fig. 5b, arrow). With increasing ring modes (1513 and 1597 cm 21 ). Other bands in
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1475
Fig. 5. SEM micrographs of pyrolytic chars prepared at 450 8C (a, b), 550 8C (c), 650 8C (d) and 750 8C (e).
the spectrum corresponded to lone aryl CH wag (855 cm21) (position below 1700 cm 21 ) or not (position above
and two-adjacent aryl CH wags (817 cm21). The band at 1700 cm21). The most characteristic bands of lignin were
1368 cm21 could be due to both OH in-plane bending and at 1513 and 1597 cm21 (aromatic ring vibrations) and
CH bending and the weak band at 667 cm21 for the out-of- between 1470 and 1460 cm21 (CH deformation and aromatic
plane OH bend. The band at 2843 cm21 represented the ring vibrations). A higher intensity of the band at 1513 cm21
symmetric CH3 stretch of the methoxyl group and the band compared to 1597 cm21 band showed that the lignin sample
at 1033 cm21 due to the C – O stretch for the O – CH3 and was probably obtained from softwood .
C – OH. The sulfonic acid groups usually appear at about The spectra for the select char samples from pyrolysis are
1200 cm21. The broad band of the hydroxyl stretching was presented in Fig. 7. Each spectrum is a spectral average of at
not used for the complete structural elucidation of the least three scans. The bands due to the atmospheric
complex lignin molecule. The same is true of the CH contributions of water vapor and CO2 have been subtracted
stretching bands at 2800 –3000 cm21 (partially also caused from the spectra in order to improve the spectral quality. A
by OH stretching) and the bands in the region of 1000 – correction was made to account for an increasingly upward
1400 cm21, caused by combination and overlapping of C – O drift in the baseline at high wave numbers. The drift was
stretching bands, and by several deformations. The width and probably due to increase in the carbonized component of the
intensity of the bands between 1000 and 1100 cm21 is char at high temperature .
dependent on presence of any sugars in the sample, while the Most bands showed a shift in the wave numbers for
bands for the hydroxyl group above 3000 cm21 are due to different chars. The exact reason for the shift was not clear.
alcoholic or phenolic components. The bands of the carbonyl The intensity of the absorbance due to the hydrogen bonded
groups appeared in the range between 1660 and 1725 cm21. OH stretching decreased with increase in pyrolysis
The exact position of the bands was dependent on whether the temperature. The decrease may be due to the loss of
CyO groups were in conjunction with the aromatic ring phenolic or alcoholic groups since the oxygen/carbon ratio
1476 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
Fig. 6. FTIR spectrum of lignin.
(O/C) ratio of the char also decreased at high temperatures, temperatures, consistent with the decrease in the oxygen
as seen in Section 3.2.4. The band due to the OH stretching containing groups at high temperatures. Since the number of
vibration was still present at 550 8C. The 13C NMR adjacent aromatic hydrogen atoms may provide an estimate
spectrum (vide infra) shows that this band must be due to of the degree of aromatic substitution and condensation, the
phenolic hydroxyls since all the aliphatic groups have variations in the aromatic CH wags in the 900 – 700 cm21
disappeared at this temperature. The broad band for the OH region were used to study the changes in the aromatic
in-plane bend also decreased with increase in temperature. structures. The band due to the aromatic ring mode
Interestingly, a band above 3600 cm21, probably corre-
sponding to free OH stretching vibrations, grew slowly with
pyrolysis temperature. This could be due to increased
carbonization of the sample. However, the intensity of the
free OH stretch was much smaller than that of the bonded
The symmetric CH3 stretch of the O – CH3 group
appeared at 2842– 2839 cm21 in chars and its intensity
decreased rapidly as the temperature increased from 250 to
400 8C. The band was completely absent above 400 8C
(Fig. 7), indicating that the CH3 groups were removed from
the substituted aromatic rings at high temperatures. A
similar behavior was shown by the OC stretch (at
1033 cm21) in the methoxyl group. The intensity of the
stretch decreased with increase in temperature. The band
due to the aliphatic CH stretch had a high intensity at 250 8C
but the intensity decreased with increase in temperature and
the band became weak at 550 8C.
The most suitable band for monitoring the aromatic
nature of the char was probably the band due to the aromatic
CH stretch. The band shifted from 3064 cm21 for lignin to
3050 cm21 for chars and its intensity increased with
temperature, especially above 300 8C. This shows that
the extent of aromatic substitution decreased at high Fig. 7. FTIR spectra of select pyrolytic chars from lignin.
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1477
decreased at high temperatures and disappeared above areas. However, due to differences in the intensities of the
400 8C. The band for the lone aryl CH wag shifted upward FTIR signals and the amounts of sample in different
to 859 cm21 at 350 8C, and to 881 cm21 at 550 8C, analysis, the concentrations and path lengths were also
suggesting a change in the chemical nature of the band. different in various analyses. As a result, only the ratios of
The locations of this band in naphthalene, substituted the peak areas with respect to a reference peak were
naphthalene, and anthracene are 875 –823, 905 – 835 and compared. The band at 1597 cm21 representing the
900 –875 cm21, respectively . It appears that fused ring aromatic ring mode was taken as the reference band and
systems were formed around 400– 500 8C. This is consistent ratios of the peak areas of various functional groups relative
with the increase of aromatic hydrogen and the loss of to the reference were calculated and compared among the
oxygen-containing functional groups. The trend for CH wag different char samples. Typical results for the OH, OCH3,
2 (at 822 cm21) paralleled that for CH wag 1 and supported aliphatic CH and aromatic ring ratios are presented in Fig. 8.
the formation of fused ring systems and an increase in Whereas the peak area ratio for the bonded OH stretch (Fig.
aromatic character around 400– 550 8C. A band, represent- 8a) was mostly greater than one, the other three ratios were
ing CH wag 3, appeared at 764 cm21 for char at 350 8C but generally below one-half. This is due to the relatively high
shifted to 758 cm21 for chars at 400 8C and above. This absorbance by the OH stretch, compared to the absorbances
band could be due to the CH wags for an aromatic ring with by the other functional groups of the unpyrolyzed lignin.
3, 4 and 5 adjacent hydrogen atoms. The shift in the band at The ratio for the bonded OH stretch was high initially but
higher temperatures suggested the formation of fused ring decreased gradually with increase in temperature, from ca.
systems, such as substituted naphthalene, anthracene, or 4.5 at 250 8C to 0.2 for char at 550 8C. The ratios of the
phenanthrene. The variations in CH wag 2 and CH wag 3 symmetric CH3 stretch (Fig. 8b) and aliphatic CH stretch
bands indicated that the extent of aromatic substitution (Fig. 8c) also decreased as the temperature increased. The
decreased as the temperature increased and that the char decrease in the symmetric CH3 stretch was sharp with the
formed a network of fused rings around 400– 500 8C. Boon ratio dropping to a negligible level at 400 8C and above. On
et al.  observed a similar increase in the aromatic the other hand, the decrease in the aliphatic CH stretch was
character and a loss of oxygen functionality for cellulose relatively slow although the ratio was small at 550 8C. A
chars at high temperatures. non-zero value for the ratio at 550 8C showed that some
The relative proportions of a functional group in different aliphatic groups, though in inﬁnitesimally small amounts,
chars may be compared in terms of the corresponding peak were still present in the char at this temperature. In contrast,
Fig. 8. Comparison of peak area ratios for the OH (a), OCH3 (b), aliphatic CH (c) and aromatic CH (d) stretches for pyrolytic chars at various temperatures. The
ratios were calculated relative to the intensity of 1597 cm21 ring mode band.
1478 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
the ratio for the aromatic CH stretch (Fig. 8d) was
essentially zero until 350 8C but increased sharply at higher
temperatures. Similar behavior was observed for the ratios
corresponding to the various CH ring wags. The results for
CH wag 1 indicated a decrease of isolated hydrogen atoms
on rings and in the extent of aromatic substitution at high
temperatures. The decrease in the isolated hydrogen atoms
may be related to the removal of peripheral groups
attached to aromatic rings and subsequent stabilization of
free radicals by the evolved hydrogen . This is
consistent with the increase in aromatic hydrogen and the
loss of oxygen-containing functional groups, OH and
OCH3. A signiﬁcant increase in intensity of CH wag 1
was observed above 400 8C, indicating the formation of
fused ring systems.
The results in Fig. 8 indicated that electron donor
substituents, such as OH and OCH3, were removed from the
substituted ring at 400 8C. The OH group in chars at high Fig. 9. 13C CPMAS NMR of alkali lignin. More detailed shift assignments
temperatures was probably attached to the aromatic group. can be found in Refs. [30–32].
Mok et al.  made similar observations from the FTIR
analysis of cellulose chars prepared at temperatures of up to before disappearing at 400 8C, as also already shown by
450 8C. The major steps were shown to be dehydration, FTIR (vide supra). From 400 to 600 8C, the aromatic-
carbonyl group formation and elimination, the decompo- bonded oxygen is progressively depleted eliminating the
sition of aliphatic units and the formation of aromatic units. phenolic and carboxyl carbons entirely. The ﬁnal product at
This indicates that at least some steps in the chemical 600 8C was an almost purely aromatic char with no organic
transformations occurring during pyrolysis may be similar
for different biomass.
The FTIR results for the oxidative chars are not presented
since they were not signiﬁcantly different from the pyrolysis
chars. The role of oxygen on the appearance and
disappearance of various functional groups was small,
probably due to the low concentration of oxygen in the
carrier gas. A similarly small effect of the oxygen on the IR
and NMR spectra was observed earlier for other biomass
materials, such as chlorogenic acid, pectin and tobacco .
3.2.4. 13C CPMAS NMR analysis
C CPMAS NMR provides a valuable complementary
method to FTIR for the characterization of char. NMR is
useful for making quantitative comparisons without resort-
ing to taking peak ratios. Rather each resonance peak can be
measured relative to the total resonance intensity to give the
relative abundance of individual molecular groups. Fig. 9
shows the 13C CPMAS NMR spectrum of the alkali lignin
with the various resonance lines assigned to different classes
of molecular substructures [31 –33]. The spectra of the
pyrolytic chars are shown in Fig. 10. The char composition
exhibited a progressive trend of depletion of lignin
substructures and increasing aromatic character as the
heating temperature increased. From 250 to 350 8C, the
aromatic, phenolic, methoxyl, hydroxymethyl and aliphatic
carbons of lignin progressively diminished to form a highly
aromatic char containing many hydroxyl, but no methoxyl
substituents. The methoxyl resonance at 55 ppm decreased
in concert with the corresponding resonance of the aromatic Fig. 10. Temperature dependence of the 13C CPMAS NMR spectra of chars
carbons bonded to the methoxyl groups at 146– 147 ppm from alkali lignin, prepared under pyrolytic conditions.
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1479
oxygen substituents, i.e. any remaining oxygen in char was comparable temperatures [16,34], this increase was more
mainly in inorganic compounds. gradual. While the pyrolysis of carbohydrates led to the
In contrast to other biomass materials such as pectin and formation of signiﬁcant numbers of carboxyl and ketone
cellulose for which pyrolysis and char formation has been groups only small amounts of these groups (not shown in
studied, lignin contains many aromatic groups that are Fig. 11) were formed from lignin. The concentration of both
native to its structure. Thus, pathways leading to the large types of carbons reached only ca. 1 –2% at ca. 400 8C and
condensed aromatic rings that form in the char can be then declined. The formation of methyl groups bonded to
expected in general to be different . Moreover, carbon reached a maximum concentration of ca. 9% at
carbohydrates undergo dehydration and elimination reac- 400 8C and then declined at higher temperatures.
tions to form unsaturated bonds and methylene carbons by The increase in aromatic character with increasing
an unknown mechanism. The number of available free pyrolysis temperature reﬂected in Fig. 11 does not
hydroxyl groups in lignin, however, is much less than in necessarily indicate the formation of new aromatic rings
carbohydrates. since the greater concentration of aromatic carbons in the
The fate of the different molecular substructures and the char can result, at least in part, from the depletion of all the
growth of the aromatic carbons in the lignin char can be remaining carbons. A more accurate assessment about
examined by measuring the 13C resonance peak areas using the formation of aromatic carbons can be made by relating
curve ﬁtting analysis, as has been done for pectin and the amount of carbon remaining in the char to the original
tobacco [34,16]. The results of this analysis are shown in sample weight. In Fig. 12, the data in Fig. 11 are scaled by
Fig. 11. Since we were concerned primarily about the trends the sample weight loss to estimate the amount of starting
and not the absolute abundance of the different molecular carbon remaining in the char. Surprisingly, the results show
substructures, we made no corrections for differences in the that, at every temperature, the number of aromatic carbons
rates of cross-polarization required for accurate spin in the char was less than in the starting material. By
counting. These corrections are nonetheless expected to be comparison, the overall number of all the non-aromatic
small. Also, we have not accounted for the resonance carbons found in the starting material was observed to
intensity in spinning side bands. However, at a spinning rate decrease exponentially with heating temperature. It appears
of 8100 Hz, the spinning side band intensity for the aromatic that, if any aromatic groups were formed from pyrolysis
carbons was only ca. 6% of the center band intensity at the reactions, some of these aromatic carbons must have
50 MHz observation frequency. Finally, because curve subsequently been lost. The concentration of aromatic
ﬁtting does not in general give a unique solution whenever carbons, however, decreased by only ca. 10% relative to the
there are multiple overlapping resonance lines, we have starting lignin even at the highest heating temperature
summed the individual resonance lines into broad classes of (600 8C). Thus, most of the weight loss was due to the
molecular substructures. volatilization of the non-aromatic carbons. These results
The graph in Fig. 11 shows that, over the entire showed that the aromatic component of the lignin was very
temperature range, the concentration of aromatic carbons resistant to thermal degradation and that the resulting char
in the char increased as all the non-aromatic carbons was highly refractory. The small weight loss at temperatures
decreased. In comparison to pectin or tobacco pyrolyzed at between 400 and 600 8C must have come from
Fig. 11. Molecular subgroup analysis obtained by curve analysis of the 13C CPMAS NMR spectra shown in Fig. 10. The results are expressed as percentage of
carbon in the char. Me ¼ methyl, Al ¼ aliphatic, OCH3 ¼ methoxyl methyl, Ar ¼ total aromatic, non-Ar ¼ all non-aromatic carbons. The sum of Ar and non-
Ar equals 100%.
1480 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
Fig. 12. Molecular subgroup analysis obtained by curve analysis of the 13C CPMAS NMR spectra. In this case, the data were obtained by scaling the percentage
carbon by the weight loss to give an estimate of the amount of starting material remaining in the char. The curve for the non-aromatic carbons was ﬁtted to a
decaying exponential function.
the elimination of phenolic type carbons. This process is
likely to result in the condensation of smaller aromatic rings
into larger PAH networks.
The 13C CPMAS NMR spectra of the samples prepared
under an oxidative atmosphere are shown in Fig. 13.
Surprisingly, at every temperature, the spectra were
virtually indistinguishable from the comparable samples
prepared under helium. This result underscores the refrac-
tory nature of the aromatic component that is resistant to
decomposition by either oxidative or pyrolysis conditions.
3.2.5. Elemental composition
The elemental compositions of the lignin chars are
presented in Fig. 14. The compositions are given as a plot of
hydrogen/carbon ratio (H/C) against the O/C in the form of
van Krevelan diagram . The results for other biomass
components, pectin and chlorogenic acid, are also shown in
the ﬁgure. The corresponding temperatures for some of the
lignin chars are given in the ﬁgure. The point with the
highest H/C ratio represents the unpyrolyzed lignin sample.
For the pyrolysis chars, the H/C ratio decreased continu-
ously with increase in temperature from ca. 1 for the char at
250 8C to 0.4 for the char at 550 8C. The O/C ratio also
decreased, from 0.3 to 0.1 in the same temperature range.
This indicates a loss of hydrogen and oxygen and a gradual
enrichment of char with carbon. The chars became
increasingly more aromatic with increase in temperature.
Since the slope of the curve decreased as the temperature
increased, especially above 350 8C, the oxygen appears to Fig. 13. Temperature dependence of the 13C CPMAS NMR spectra of chars
be lost more rapidly relative to hydrogen at high from alkali lignin, prepared in an oxidative atmosphere containing 5% O2 in
temperatures. This indicates some decarboxylation at high helium.
R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 1481
Fig. 15. Reactivity of char to form PAHs at 700 8C. The char was prepared
at 300 8C (Char #1), 350 8C (Char #2), and 400 8C (Char #3).
Fig. 14. van Krevelan diagram for lignin chars. concentration but benzo[a]pyrene (BaP) was not observed.
This showed that BaP was either not formed or had
temperatures. The predominant reaction was dehydration at
negligibly low yield. The difference in reactivity may be due
low temperatures followed by dehydration and some
to the differences in the physical and chemical character-
decarboxylation at high temperatures. The results indicate
istics of the chars. The 300 8C-char has the lowest surface
that the extent of direct dehydrogenation or demethanation
area (Fig. 3). It also has the lowest aromatic carbon content
was small. This is in contrast to the gaseous product analysis
(Fig. 11). On the other hand, the aliphatic carbon content
that showed the presence of hydrogen in the product at
was higher for this char compared to Char #2 and Char #3
(Figs. 8 and 11). The reactivity of Char #3 is only slightly
It is interesting to note that the compositions of the lignin
higher than Char #2 although Char #3 is relatively more
chars above 400 8C were similar to those for the chars from
aromatic in nature (Fig. 11). The surface areas of these two
pectin or chlorogenic acid. This showed that the nature of
chars are comparable. Thus, based on these limited results, it
substrate had only a small effect on the composition of char
appears that surface area and aromaticity of the char may be
at high temperatures although the pyrolysis temperatures to
important factors in the PAH formation. A char with
obtain a given composition might be different for different
essentially no surface area may give relatively low PAH
substrates. Fig. 14 also shows that some oxygen remained in
yields. An increase in the aromaticity of the char may
lignin chars even at high temperatures. The NMR (presented
increase the reactivity towards PAHs initially but a highly
earlier) showed that the remaining oxygen was mainly
aromatic and carbonized char may not necessarily have a
phenolic. Signiﬁcant oxygen is also expected to be present
high propensity for the PAH formation. These observations
in inorganic salts as oxides. The chars from the oxidative
are consistent with the results from the other biomass
pyrolysis showed a similar composition, an observation also constituents, such as chlorogenic acid, which gave con-
made earlier with chlorogenic acid and pectin [34,35]. The siderably higher PAH yields than lignin . Although
results are also consistent with the composition of the lignin is aromatic, its structure is highly cross-linked and
volatile product that comprised mainly of CO, CO2, and many of its native aromatic groups appear to be refractory
water, with concentrations of the organic components being and resistant to initial degradation. The low reactivity is also
relatively small. indicated by the high ultimate char yield obtained from
3.3. Char reactivity
4. Summary and conclusions
Fig. 15 shows the reactivity of three different chars. The
reactivity is given in terms of total concentration of PAHs in The results in this study showed that both the yield and
tar obtained by pyrolyzing the char at 700 8C. The chars characteristics of chars from lignin were dependent on the
were prepared by pyrolysis at 300 8C (Char #1), 350 8C pyrolysis conditions. Up to 40% of lignin was obtained as
(Char #2) and 400 8C (Char #3). The reactivity of Char #1 is char after pyrolysis at 750 8C. The char yield decreased
lower than that of the other two chars. The principal PAHs rapidly in the oxidizing atmosphere. The high char yields
in each case were two- and three-ring PAHs, such as were probably due to the presence of inorganic components
acenaphthylene, ﬂuorene, phenanthrene, anthracene and such as sodium and potassium in lignin. The partial removal
ﬂuoranthene. Four-ring PAHs were observed in small of these inorganic components has been observed to
1482 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482
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