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  • 1. Fuel 83 (2004) 1469–1482 www.fuelfirst.com 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 Abstract 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 [1]. 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 [2] 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. [3] 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: w.geoffrey.chan@pmusa.com (W.G. Chan). formation of phenols occurred in the 500 –600 8C region. The 1 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. doi:10.1016/j.fuel.2003.11.015
  • 2. 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. [4] 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 [17]. 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. [5] 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. [6] 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. [4]. Iatridis and 750 8C were used. The chemical composition of the char Gavalas [7] 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 13 Britt et al. [8] reported that lignin pyrolysis occurred C nuclear magnetic resonance spectroscopy (CPMAS 13C mainly by a free-radical reaction mechanism. Jakab et al. [9] 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).
  • 3. 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 [16]. 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.
  • 4. 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 [16], 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 [1] 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 [18] 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. [4] 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 [7] and pectin [34] 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 [15]. 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 [19]. 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 [20]. 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
  • 5. 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 [23] 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. [24] 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.
  • 6. 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. [25] 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. [26] 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 [26]. 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
  • 7. 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 [1]. 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 [27]. 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
  • 8. 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 OH stretch. 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.
  • 9. 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 [1]. 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. [28] 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.
  • 10. 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 [29]. 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. [30] 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 [16]. 3.2.4. 13C CPMAS NMR analysis 13 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.
  • 11. 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 [16]. 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%.
  • 12. 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 [17]. 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.
  • 13. 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 #3 550 8C. (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 [17]. 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 lignin. 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
  • 14. 1482 R.K. Sharma et al. / Fuel 83 (2004) 1469–1482 decrease the char yield and the PAH formation. The References maximum surface area of the char occurred at 350 – 400 8C and appeared to be associated with the completion [1] Fengel D, Wegener G. Wood: chemistry ultrastructure, reactions. New York: Walter de Gruyter; 1984. p. 132 –181. of the solidification stage within the char. The ensuing [2] Nimz H. Angew Chem Int Ed 1974;13:313. carbonization step at high temperatures was detrimental to [3] Schlotzauer WS, Schmeltz I, Hickey LC. Tob Sci 1967;11:31. the development of a porous structure in the char. The [4] Caballero JA, Font R, Marcilla A. J Anal Appl Pyrol 1996;36:159. surface area was low, contrary to the SEM results that [5] Alen R, Kuoppala E, Oesch P. J Anal Appl Pyrol 1996;36:137. suggested a high surface area due to the presence of fine [6] Nunn TR, Howard JB, Longwell JP, Peters WA. 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